Waltham, Mass. ? Jim O’Brien quit as coach of the Boston Celtics Tuesday, after half a season under new boss Danny Ainge.

Ainge said O’Brien told him he wanted to resign at a meeting Tuesday morning, one of their regularly scheduled sit-downs to discuss “philosophical differences” on running the team.

“The philosophical differences, I thought, were much smaller than Jim thought,” said Ainge, who announced O’Brien’s departure at a news conference at the team’s practice facility.

Assistant coach John Carroll will take over for the rest of the season, Ainge said. Another assistant, Dick Harter, was let go.

Ainge said he took a more long-term view of building the team, while O’Brien was more concerned with short-term results.

“Jim did not see that long-term vision that I saw,” said Ainge, who took over as the team’s director of basketball operations during last season’s playoffs.

Ainge, who appeared at the news conference with one of the team’s owners, Wyc Grousbeck, also was trying to add more offense to the team, while O’Brien always had placed a heavy emphasis on defense.

“He was not sure he’s the man for that job … he didn’t want to take our money and our time under false pretenses,” Grousbeck said.