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                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549
                                  SCHEDULE 13G
                   Under the Securities Exchange Act of 1934
                             (Amendment No._____)*


                                 OSI SYSTEM INC.
--------------------------------------------------------------------------------
                                (Name of Issuer)

                       COMMON CIK 0001039065 IRS 330238801
--------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                    671044105
                         ------------------------------
                                 (CUSIP Number)

                                    12/31/02
--------------------------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:
     [ ] Rule 13d-1(b)
     [X] Rule 13d-1(c)
     [_] Rule 13d-1(d)
*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.
The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).

                               Page 1 of 6 pages



                                  SCHEDULE 13G
===============================================================================
  CUSIP NO. 671044105
           -----------
------------------------------------------------------------------------------
      NAMES OF REPORTING PERSONS.
 1    I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY).

      Essex Investment Management Company, LLC 04-3409177
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
 2                                                              (a) [_]
                                                                (b) [_]
------------------------------------------------------------------------------
      SEC USE ONLY
 3
------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4
      Massachusetts
------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5
     NUMBER OF            1,217,130
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6
     OWNED BY             0
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7
    REPORTING             1,302,312
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8
                          0
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9
      1,302,312
------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES
10    (SEE INSTRUCTIONS)
                                                                    [_]
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
11
      9.02%
------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)
12
      IA
------------------------------------------------------------------------------

                               Page 2 of 6 pages



                          INSTRUCTIONS FOR SCHEDULE 13G

Instructions for Cover Page

(1)  Names and Social Security Numbers of Reporting Persons-Furnish the full
     legal name of each person for whom the report is filed-i.e., each person
     required to sign the schedule itself-including each member of a group. Do
     not include the name of a person required to be identified in the report
     but who is not a reporting person. Reporting persons are also requested to
     furnish their Social Security or I.R.S. identification numbers, although
     disclosure of such numbers is voluntary, not mandatory (see "SPECIAL
     INSTRUCTIONS FOR COMPLYING WITH SCHEDULE 13G," below).

(2)  If any of the shares beneficially owned by a reporting person are held as a
     member of a group and such membership is expressly affirmed, please check
     row 2(a). If the membership in a group is disclaimed or the reporting
     person describes a relationship with other persons but does not affirm the
     existence of a group, please check row 2(b) [unless a joint filing pursuant
     to Rule 13d-1(e)(1) in which case it may not be necessary to check row
     2(b)].

(3)  The third row is for SEC internal use; please leave blank.

(4)  Citizenship or Place of Organization-Furnish citizenship if the named
     reporting person is a natural person. Otherwise, furnish place of
     organization.

(5)-(9), (11) Aggregate Amount Beneficially Owned by Each Reporting Person,
     Etc.-Rows (5) through (9) inclusive, and (11) are to be completed in
     accordance with the provisions of Item 4 of Schedule 13G. All percentages
     are to be rounded off to the nearest tenth (one place after decimal point).

(10) Check if the aggregate amount reported as beneficially owned in row (8)
     does not include shares as to which beneficial ownership is disclaimed
     pursuant to Rule 13d-4 [17 CFR 240.13d-4] under the Securities Exchange Act
     of 1934.

(12) Types of Reporting Person-Please classify each "reporting person" according
     to the following breakdown (see Item 3 of Schedule 13G) and place the
     appropriate symbol on the form:

                Category                                  Symbol
           Broker-Dealer                                    BD
           Bank                                             BK
           Insurance Company                                IC
           Investment Company                               IV
           Investment Adviser                               IA
           Employee Benefit Plan, Pension Fund,
             or Endowment Fund                              EP
           Parent Holding Company                           HC
           Corporation                                      CO
           Partnership                                      PN
           Individual                                       IN
           Other                                            OO

Notes:
     Attach as many copies of the second part of the cover page as are needed,
one reporting person per page.

     Filing persons may, in order to avoid unnecessary duplication, answer items
on the schedules (Schedule 13D, 13G or 14D-1) by appropriate cross references to
an Item or items on the cover page(s). This approach may only be used where the
cover page item or items provide all the disclosure required by the schedule
item. Moreover, such a use of a cover page item will result in the item becoming
a part of the schedule and accordingly being considered as "filed" for purposes
of Section 18 of the Securities Exchange Act or otherwise subject to the
liabilities of that section of the Act.

     Reporting persons may comply with their cover page filing requirements by
filing either completed copies of the blank forms available from the Commission,
printed or typed facsimiles, or computer printed facsimiles, provided the
documents filed have identical formats to the forms prescribed in the
Commission's regulations and meet existing Securities Exchange Act rules as to
such matters as clarity and size (Securities Exchange Act Rule 12b-12).

              SPECIAL INSTRUCTIONS FOR COMPLYING WITH SCHEDULE 13G

     Under Sections 13(d), 13(g), and 23 of the Securities Exchange Act of 1934
and the rules and regulations thereunder, the Commission is authorized to
solicit the information required to be supplied by this schedule by certain
security holders of certain issuers.

     Disclosure of the information specified in this schedule is mandatory,
except for Social Security or I.R.S. identification numbers, disclosure of which
is voluntary. The information will be used for the primary purpose of
determining and disclosing the holdings of certain beneficial owners of certain
equity securities. This statement will be made a matter of public record.
Therefore, any information given will be available for inspection by any member
of the public.

                                Page 3 of 6 pages



     Because of the public nature of the information, the Commission can utilize
it for a variety of purposes, including referral to other governmental
authorities of securities self-regulatory organizations for investigatory
purposes or in connection with litigation involving the Federal securities laws
or other civil, criminal or regulatory statutes or provisions, Social Security
or I.R.S. identification numbers, if furnished, will assist the Commission in
identifying security holders and, therefore, in promptly processing statements
of beneficial ownership of securities.

     Failure to disclose the information requested by this schedule, except for
Social Security or I.R.S. Identification numbers, may result in civil or
criminal action against the persons involved for violation of the Federal
securities laws and rules promulgated thereunder.

                              GENERAL INSTRUCTIONS

A.   Statements containing the information required by this schedule shall be
filed not later than February 14 following the calendar year covered by the
statement or within the time specified in Rule 13d-1(b)(2), if applicable.

B.   Information contained in a form which is required to be filed by rules
under section 13(f)(15 U.S.C. 78m(f)) for the same calendar year as that covered
by a statement on this schedule may be incorporated by reference in response to
any of the items of this schedule. If such information is incorporated by
reference in this schedule, copies of the relevant pages of such form shall be
filed as an exhibit to this schedule.

C.   The item numbers and captions of the items shall be included by the text of
the items is to be omitted. The answers to the items shall be so prepared as to
indicate clearly the coverage of the items without referring to the text of the
items. Answer every item. If an item is inapplicable or the answer is in the
negative, so state.

Item 1
     (a) Name of Issuer OSI SYSTEM Inc.
     (b) Address of Issuer's Principal Executive Offices 12525 Chardon Avenue,
         Hawthorne, CA 90250
Item 2
     (a) Name of Person Filing Essex Investment Management Company, LLC.
     (b) Address of Principal Business Office or, if none, Residence
         125 High Street, 29th FL, Boston, MA 02110
     (c) Citizenship U.S.A.
     (d) Title of Class of Securities Common
     (e) CUSIP Number 671044105
Item 3. If this statement is filed pursuant to (S)(S)240.13d-1(b) or
        240.13d-2(b) or (c), check whether the person filing is a:
     (a) [_]  Broker or dealer registered under section 15 of the Act
              (15 U.S.C. 78o).
     (b) [_]  Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).
     (c) [_]  Insurance company as defined in section 3(a)(19) of the Act
              (15 U.S.C. 78c).
     (d) [X]  Investment company registered under section 8 of the Investment
              Company Act of 1940 (15 U.S.C. 80a-8).
     (e) [_]  An investment adviser in accordance with
              (S)240.13d-1(b)(1)(ii)(E);
     (f) [_]  An employee benefit plan or endowment fund in accordance with
              (S)240.13d-1(b)(1)(ii)(F);
     (g) [_]  A parent holding company or control person in accordance with
              (S)240.13d-1(b)(1)(ii)(G);
     (h) [_]  A savings associations as defined in Section 3(b) of the Federal
              Deposit Insurance Act (12 U.S.C. 1813);
     (i) [_]  A church plan that is excluded from the definition of an
              investment company under section 3(c)(14) of the Investment
              Company Act of 1940 (15 U.S.C. 80a-3);
     (j) [_]  Group, in accordance with (S)240.13d-1(b)(1)(ii)(J).
Item 4. Ownership.
     Provide the following information regarding the aggregate number and
percentage of the class of securities of the issuer identified in Item 1.
     (a)  Amount beneficially owned: 1,833,067
     (b)  Percent of class: 14.34%
     (c)  Number of shares as to which the person has:
          (i)   Sole power to vote or to direct the vote 1,508,005
          (ii)  Shared power to vote or to direct the vote 0
          (iii) Sole power to dispose or to direct the disposition of  1,833,067
          (iv)  Shared power to dispose or to direct the disposition of  0
     Instruction. For computations regarding securities which represent a right
to acquire an underlying security see (S)240.13d-3(d)(1).

                               Page 4 of 6 pages



===============================================================================

Item 5.  Ownership of Five Percent or Less of a Class.
     If this statement is being filed to report the fact that as of the date
hereof the reporting person has ceased to be the beneficial owner of more than
five percent of the class of securities, check the following: [_]
Instruction: Dissolution of a group requires a response to this item.
Item 6.  Ownership of More than Five Percent on Behalf of Another Person.
     If any other person is known to have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the sale of, such
securities, a statement to that effect should be included in response to this
item, and if such interest relates to more than five percent of the class, such
person should be identified. A listing of the shareholders of an investment
company registered under the Investment Company Act of 1940 or the beneficiaries
of the employee benefit plan, pension fund or endowment fund is not required.
Item 7.  Identification and Classification of the Subsidiary Which Acquired the
         Security Being Reported on by the Parent Holding Company.
     If a parent holding company has filed this schedule, pursuant to Rule
13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating
the identity and the Item 3 classification of the relevant subsidiary. If a
parent holding company has filed this schedule pursuant to Rule 13d-1(c) or Rule
13d-1(d), attach an exhibit stating the identification of the relevant
subsidiary.
Item 8.  Identification and Classification of Members of the Group.
     If a group has filed this schedule pursuant to (S)240.13d-1(b)(1)(ii)(J),
so indicate under Item 3(j) and attach an exhibit stating the identity and Item
3 classification of each member of the group. If a group has filed this schedule
pursuant to (S)240.13d-1(c) or (S)240.13d-1(d), attach an exhibit stating the
identity of each member of the group.
Item 9.  Notice of Dissolution of a Group
     Notice of dissolution of a group may be furnished as an exhibit stating
the date of the dissolution and that all further filings with respect to
transactions in the security reported on will be filed, if required, by members
of the group, in their individual capacity. See Item 5.
Item 10. Certification
     (a)  The following certification shall be included if the statement is
filed pursuant to (S)240.13d-1(b):
               By signing below I certify that, to the best of my knowledge and
               belief, the securities referred to above were acquired and are
               held in the ordinary course of business and were not acquired and
               are not held for the purpose of or with the effect of changing or
               influencing the control of the issuer of the securities and were
               not acquired and are not held in connection with or as a
               participant in any transaction having that purpose or effect.
     (b)  The following certification shall be included if the statement is
filed pursuant to (S)240.13d-1(c):
               By signing below I certify that, to the best of my knowledge and
               belief, the securities referred to above were not acquired and
               are not held for the purpose of or with the effect of changing or
               influencing the control of the issuer of the securities and were
               not acquired and are not held in connection with or as a
               participant in any transaction having that purpose or effect.

                               Page 5 of 6 pages



===============================================================================

                                   SIGNATURE
     After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.

                                                      January 13, 2003
                                              ----------------------------------
                                                            Date

                                                /s/ Michael S. McCarthy
                                              ----------------------------------
                                                          Signature

                                                     Michael S. McCarthy--
                                                 Vice President of Operations
                                              ----------------------------------
                                                          Name/Title
     The original statement shall be signed by each person on whose behalf the
statement is filed or his authorized representative. If the statement is signed
on behalf of a person by his authorized representative other than an executive
officer or general partner of the filing person, evidence of the
representative's authority to sign on behalf of such person shall be filed with
the statement, provided, however, that a power of attorney for this purpose
which is already on file with the Commission may be incorporated by reference.
The name and any title of each person who signs the statement shall be typed or
printed beneath his signature.
NOTE: Schedules filed in paper format shall include a signed original and five
copies of the schedule, including all exhibits. See (S)240.13d-7 for other
parties for whom copies are to be sent.
     Attention: Intentional misstatements or omissions of fact constitute
Federal criminal violations. (See 18 U.S.C. 1001.)

                               Page 6 of 6 pages