UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

(Amendment No. 4)

Under the Securities Exchange Act of 1934

OncoGenex Pharmaceuticals, Inc.

(Name of Issuer)

Common Stock, $0.001 par value

(Title of Class of Securities)

68230A106

(CUSIP Number)

December 31, 2012

(Date of Event which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule if filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

CUSIP No. 68230A106

CUSIP No. 68230A106

CUSIP No. 68230A106

CUSIP No. 68230A106

CUSIP No. 68230A106

CUSIP No. 68230A106

This Amendment No. 4 (“Amendment No. 4”) amends and supplements the statement on Schedule 13G initially filed on June 10, 2009 (the “Original Filing”), as amended on February 16, 2010 (“Amendment No. 1”), November 1, 2010 (“Amendment No. 2”) and February 14, 2012 (“Amendment No. 3”). The Original Filing remains in effect except to the extent that it is amended, restated or superseded by information contained in Amendment No. 1, Amendment No. 2, Amendment No. 3 or this Amendment No. 4. Capitalized terms used and not defined in this Amendment No. 4 have the meanings set forth in the Original Filing, Amendment No. 1, Amendment No. 2 or Amendment No. 3.

Item 4. Ownership.

Item 4 is hereby restated as follows:

Boxer Capital, Boxer Management and Joseph Lewis beneficially own 260,000* Common Shares, the sum of 100,000 Common Shares and 160,000 Common Shares underlying currently exercisable warrants to purchase Common Shares (the “Warrants”). MVA beneficially owns 15,000* Common Shares, the sum of 10,000 Common Shares and 5,000 Common Shares underlying Warrants. Neil Reisman beneficially owns 2,000* Common Shares. Ivan Lieberburg no longer beneficially owns any Common Shares.

The Common Shares beneficially owned by Boxer Capital, Boxer Management and Joseph Lewis represent 1.8% of the Issuer’s outstanding Common Shares. The Common Shares beneficially owned by MVA represent 0.1% of the Issuer’s outstanding Common Shares. The Common Shares beneficially owned by Neil Reisman represent 0.01% of the Issuer’s outstanding Common Shares.

MVA has the sole power to vote the 15,000* Common Shares it beneficially owns. Neil Reisman has the sole power to vote the 2,000* Common Shares he beneficially owns. Boxer Capital, Boxer Management and Joseph Lewis do not have the sole power to dispose or direct the disposition of any Common Shares.

Boxer Capital, Boxer Management and Joseph Lewis have shared voting power with respect to the 260,000* Common Shares they beneficially own. MVA and Neil Reisman do not have shared power to vote or direct the vote of any Common Shares.

MVA has the sole power to dispose of the 15,000* Common Shares it beneficially owns. Neil Reisman has the sole power to dispose of the 2,000* Common Shares he beneficially owns. Boxer Capital, Boxer Management and Joseph Lewis do not have the sole power to dispose or direct the disposition of any Common Shares.

Boxer Capital, Boxer Management and Joseph Lewis have shared dispositive power with respect to the 260,000* Common Shares they beneficially own. MVA and Neil Reisman do not have shared power to dispose of or direct the disposition of any Common Shares. Ivan Lieberburg does not own any Common Shares.

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 Persons have ceased to be the beneficial owner of more than 5 percent of the class of securities, check the following x.

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

Item 6 is restated as follows:

Only Boxer Capital has the right to receive dividends and the proceeds from sale of the Common Shares held by the Boxer Capital. Only MVA has the right to receive dividends and the proceeds from sale of the Common Shares held by the MVA. Only Mr. Reisman has the right to receive dividends and the proceeds from sale of the Common Shares held by the him. See Item 4 above.

Item 10. Certification.

(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 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.

Exhibits

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.