McGary, 24, appeared in just 20 games last season for the Thunder after dealing with injury issues, including a concussion in training camp. He was also away from the team for a couple of weeks at the end of the regular season because of a "personal matter" that he declined to discuss.
"I clearly made a mistake and for that I apologize to the organization, my teammates, and our fans," McGary said in a statement. "I realize there are only so many opportunities given to you and I need make sure that as a person and as a player, I take accountability and ensure that this does not happen again."
McGary was suspended a year while in college at Michigan for testing positive for marijuana, but avoided the suspension by entering the NBA draft.
McGary was taken 21st overall by the Thunder and made a strong impression his rookie season, posting three double-doubles in 32 games.
McGary averaged 1.3 points and 0.9 rebounds last season in 3.6 minutes.