The move Thursday comes a day after Gradkowski expressed hope that he could return from his second shoulder injury before the end of the season. But Coach Tom Cable says it is unlikely Gradkowski would have been able to return before then.
O'Sullivan was last on San Diego's roster before being cut in October. Oakland plays the Chargers this week.
This is O'Sullivan's 11th team since being drafted by New Orleans in 2002. He has played 17 career games, making eight starts for San Francisco in 2008.