Cheeks will be the 76ers 21st head coach. He takes over in Philadelphia after more than three seasons as head coach of the Portland Trail Blazers.
O'Brien, a 53year-old Philadelphia native, lasted one season in Philadelphia. He directed the 76ers to a 43-39 record.
O'Brien spent seven seasons in the Boston organization. He took over in Boston for Rick Pitino in January 2001 and had a head coaching record of 139-119 with the Celtics. He resigned as Celtics coach in January 2004.
Cheeks, 49, was fired as coach of the Trail Blazers March 2 after the club dropped 33 of its first 55 games. He was named Portland coach June 27, 2001 after seven years as an assistant in Philadelphia.
Cheeks was a four-time NBA All-Star during his 15-year playing career.