Sec Form 3 Filing - ONEX CORP @ ATI Physical Therapy, Inc. - 2023-06-15

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person ONEX CORP 2. Issuer Name and Ticker or Trading Symbol ATI Physical Therapy, Inc. [ ATIP] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 161 BAY STREET, P.O. BOX 700 3. Date of Earliest Transaction (MM/DD/YY) 06/15/2023 (Street) TORONTO, A6M5J 2S1 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Series A Preferred Stock 20,000 I See Footnote ( 1 ) ( 8 ) Series B Preferred Stock 9,126 I See Footnote ( 2 ) ( 8 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series I Warrants (right to acquire) ( 3 ) ( 3 ) ( 3 ) Class A Common Stock 13,240 I See Footnotes ( 3 ) ( 8 ) Series II Warrants (right to acquire) ( 4 ) ( 4 ) ( 4 ) Class A Common Stock 14,634 I See Footnotes ( 4 ) ( 8 ) Convertible PIK Notes ( 5 ) ( 6 ) ( 7 ) ( 5 )( 6 )( 7 ) ( 5 )( 6 )( 7 ) Class A Common Stock 730,166 ( 7 ) I See Footnotes ( 5 ) ( 6 ) ( 7 ) ( 8 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ONEX CORP 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X OMI Partnership Holdings Ltd. 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X Onex Capital Solutions GP, LLC 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X Onex Capital Solutions GP, LP 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X Onex Capital Solutions Holdings, LP 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X SCHWARTZ GERALD W 161 BAY STREET P.O. BOX 700 TORONTO, A6M5J 2S1 X Signatures Onex Corporation, by: /s/ Andrea Daly, Managing Director--General Counsel 06/26/2023 Signature of Reporting Person Date OMI Partnership Holdings Ltd., by: /s/ Steve Gutman, Authorized Signatory 06/26/2023 Signature of Reporting Person Date Onex Capital Solutions GP, LLC, by: /s/ Steve Gutman, General Counsel 06/26/2023 Signature of Reporting Person Date Onex Capital Solutions GP, LP, by: /s/ Onex Capital Solutions GP, LLC, its general partner 06/26/2023 Signature of Reporting Person Date Onex Capital Solutions Holdings, LP, by: /s/ Onex Capital Solutions GP, LP, its general partner and Onex Capital Solutions GP, LLC, its general partner 06/26/2023 Signature of Reporting Person Date Gerald W. Schwartz, by: Andrea Daly, Attorney-in-fact 06/26/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Reflects shares of Series A Preferred Stock of the Issuer held by Onex Capital Solutions Holdings, LP ("OCS"). ( 2 )Reflects shares of Series B Preferred Stock of the Issuer held by OCS. ( 3 )Reflects warrants to acquire shares of Class A Common Stock of the Issuer ("Common Stock") held by OCS ("Series I Warrants"). The Series I Warrants may be exercised by at any time prior to their expiration on February 24, 2027. The exercise price of the Series I Warrants is $150.00 per share. ( 4 )Reflects warrants to acquire shares of Common Stock held by OCS ("Series II Warrants"). The Series II Warrants may be exercised at any time prior to their expiration on February 24, 2027. The exercise price of the Series II Warrants is $0.50 per share. ( 5 )Reflects senior second lien secured convertible PIK notes held by OCS ("Notes"). The aggregate principal amount of the Notes is $9,127,076.88, and the Notes accrue interest at a rate of 8.00% per annum. Subject to earlier conversion in accordance with their terms, the Notes will become due and payable on August 24, 2028 (the "Maturity Date") in an amount equal to the principal amount of the Notes outstanding on such date (plus any accrued but unpaid interest thereon). ( 6 )All or any portion of the Notes may be converted at the election of the Reporting Persons at any time prior to the close of business on the business day immediately preceding the Maturity Date into a number of shares of Common Stock, determined by dividing (a) the outstanding balance of the principal amount of the Notes being converted (together with the accrued but unpaid interest thereon) as of the applicable conversion date, by (b) a conversion price of $12.50, which such conversion price is subject to certain anti-dilution adjustments in accordance with their terms. ( 7 )This number represents the number of shares of Common Stock issuable upon conversion of the Notes if the Reporting Persons elect to convert the Notes based on the principal amount of the Notes (as described in footnote (5) above) and the conversion price (as described in footnote (6) above). ( 8 )Onex Corporation ("Onex") owns all of the equity of OMI Partnership Holdings Ltd. ("OMI"), which is the sole member of Onex Capital Solutions GP, LLC ("OCS GP LLC"), which is the general partner of Onex Capital Solutions GP, LP ("OCS GP LP"), which is the general partner of OCS. Accordingly, Onex may be deemed to beneficially own the reported securities held by OCS. [Gerald W. Schwartz, the Chairman of Onex, indirectly owns shares representing a majority of the voting rights of the shares of Onex, and as such may be deemed to beneficially own all of the reported securities beneficially owned by Onex.] Each Reporting Person disclaims beneficial ownership of the reported securities except to the extent of his or its pecuniary interest therein.

Remarks: Exhibit 99 - Joint Filer Statement

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