Ricky Davis had a playoff career-high 22 points and Pierce added 20 for Boston. O'Neal led Indiana with 26 points.
"We didn't expect to lose this game. It's another tough obstacle for us," O'Neal said. "But nobody's in a panic mode. We've got one more chance to move on. Nobody said it would be easy."
The winner of Game 7 on Saturday will open the Eastern Conference semifinals at defending NBA champion Detroit.
"I think Reggie might have just played his last game in his building," Walker said in a postgame interview with Turner Sports.
Walker had 13 of his 24 points in the fourth quarter and overtime, but it was Boston's bench that initially put the Celtics in position to win.
Led by Davis and Jefferson, who finished with 11 points and 14 rebounds, the Celtics' bench outscored the starters in the first half to overcome Indiana's early lead. The Pacers scored the first 10 points of the game, and a basket and free throw by Miller gave Indiana its biggest lead, 19-8.
The Celtics came back in the second quarter, led 49-43 at the half and 70-62 after three quarters before consecutive 3-pointers by Indiana's James Jones and Fred Jones started the frantic fourth quarter.
Rockets 101, Mavericks 83
In Houston, Playing with the desperation of a man on the brink of elimination, Tracy McGrady had 37 points, eight rebounds and seven assists to help the Houston Rockets avoid elimination with a 101-83 victory over Dallas on Thursday.
McGrady refused to be denied in Game 6, and now he'll have one more chance to advance in the playoffs for the first time in his career.
The first-round series is now tied 3-3, with Game 7 in Dallas on Saturday. The winner will face top-seeded Phoenix in the Western Conference semifinals.
Mike James came off the bench for 22 points and Jon Barry scored 12 of his 14 points in the fourth quarter for the Rockets, who have been bounced in the opening round in their last three playoff appearances. Houston last won a series in 1997, when the lineup featured Hakeem Olajuwon, Clyde Drexler and Charles Barkley.
Jerry Stackhouse led Dallas with 21 points, and Dirk Nowitzki scored 19 on only 5-of-22 shooting as he continued his series-long struggles against McGrady's defense.
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