The Celtics signed coach Jim O'Brien to a two-year extension yesterday, which means he is now under contract for the next three seasons. Although terms of the deal were not disclosed, sources close to the team said O'Brien will be paid $3 million per year when the extension goes into effect for the 2004-05 season. O'Brien will earn $1.5 million next season.

O'Brien anticipates his entire coaching staff will return for next season. Executive director of basketball operations Danny Ainge said completing new contracts for the assistants was his ''next order of business'' and he also anticipates ''no problems of having the entire staff back.'' According to Ainge, the determination to give O'Brien a two-year extension came after a brief, but intense period of evaluation. No other coaches were contacted.