Fisher joins Oklahoma City for the stretch run for the second straight season. He played in 20 regular-season games and then all 20 playoff games last season.
Fisher appeared in nine games this season with the Dallas Mavericks before asking for his release in December because the 38-year-old -- after suffering a knee injury -- wanted to return home to his family in Los Angeles.
Fisher won five NBA championships with the Los Angeles Lakers before getting traded to Houston last season, then buying his way out of his contract with the Rockets.
Fisher is also the president of the NBA Players Association, which recently ousted longtime executive director Billy Hunter.
Fisher, who won five championships with the Lakers, was viewed as a key locker room presence by Oklahoma City coach Scott Brooks as the Thunder made it all the way to the Finals, losing to the Miami Heat in five games.
Fisher was productive during his brief stint in Dallas, averaging 8.6 points and 3.6 assists in just more than 25 minutes per game. And he was a natural fit in the Oklahoma City locker room last spring, delivering valuable championship experience to a team short on playoff-tested veterans.
Upon his release from the Mavericks in December, Fisher said: "The recovery time [from the knee injury] will be approximately two weeks. With this news and the difficulty I have been having being away from my family, I have asked the organization to waive me so I can return home.
"[Mavericks owner] Mark Cuban has been extremely supportive and has granted me this request. I cannot say enough good things about this organization. From Mark, to Coach [Rick] Carlisle, to the players on the Mavericks' team, I sincerely thank them for the opportunity.
"I have made decisions in the past, leaving money and opportunity on the table, and I will need to do that again. My family is my priority and that is where I choose to be. I won't close the possibility that I will play again; however, for now my family and being close to them remains the priority."
Information from The Associated Press, ESPN.com's Marc Stein, and ESPNLosAngeles.com's Dave McMenamin and Ramona Shelburne contributed to this report.