Thabeet's agent, Bill Duffy, confirmed Thursday the 7-foot-3 center has agreed to a two-year guaranteed contract with the Thunder.
NBA teams aren't allowed to announce new signings until Wednesday.
Thabeet was the No. 2 pick in the 2009 draft by Memphis but has averaged only 2.2 points and 2.7 rebounds in limited playing time over the past three seasons with the Grizzlies, Houston and Portland.
Information from The Associated Press was used in this report.