BOSTON (AP) Jim O'Brien quit as coach of the Boston Celtics on Tuesday after half a season under new boss Danny Ainge, a team source told The Associated Press.

The team said it would have a ``major announcement'' but did not say why. The source said on condition of anonymity O'Brien's departure would be announced.

The Celtics are 22-24 after reaching the playoffs the past two seasons.

Ainge made numerous changes, trading Antoine Walker to Dallas before the season and Eric Williams and Tony Battie to Cleveland on Dec. 15. The first deal deprived the Celtics of a second scoring threat to go along with Paul Pierce. The second took away two veteran leaders and keys to O'Brien's defense-oriented approach.

After taking over, Ainge gave O'Brien a two-year contract extension running through the 2005-06 season. O'Brien became coach when Rick Pitino resigned in January 2001.

Under O'Brien, the Celtics reached the Eastern Conference finals in 2002 and the conference semifinals in 2003. They lost both times to New Jersey.