And they kept the Detroit Pistons winless with a 96-85 win in their home opener on Friday night.
O'Neal, nearly invisible in the first three games, and Pierce, who missed all of them with a bruised right heel, got the Celtics back in rhythm as they had their starting five intact for the first time.
O'Neal led Boston with 19 points after playing passively in totaling just eight points in the other games.
What was the difference?
"Playing extended minutes," he said. "When you can get a rhythm, you can get a flow and you can get comfortable with what the team is doing. If you look at the first couple of games, I was in and out so fast that it was almost impossible to get any flow to the game."
O'Neal was aggressive throughout, scoring 10 points in the first half, which ended with Boston on top 50-43. Then he scored seven in the third quarter when the Celtics outscored the Pistons 36-21 to take an 86-64 lead.
"He gave us a great lift, played with great energy," Celtics coach Doc Rivers said. "I thought he was really focused on setting picks and rebounding and doing all the little things."
The closest the Pistons came in the fourth quarter was the 11-point final margin.
Pierce, who had taken part in just one Celtics practice, finished with 12 points. More than that, he brought a fiery attitude and helped the Celtics stay spaced properly on the floor. He drew defensive attention with his shooting ability, opening opportunities for teammates.
"It felt good to finally get out there," Pierce said. "I was expecting to be me and I don't settle for anything less. I worked tremendously hard. I did a lot of things. I did what I could do coming into the game, as much as I could, other than practice. I just stayed focused."
Ray Allen and Brandon Bass added 17 points apiece for Boston. O'Neal, injured much of last season, his first with the Celtics, had seven rebounds, blocked two shots and sank seven of nine shots after making three of 12 in his first three games.
"Defensively and rebounding is how I'm going to judge myself," he said. "That's my role. I've heard people ask me, 'Why would you accept it?' But that's what you do on a championship-caliber team."
"We just have to work and be a whole lot more efficient offensively," said Detroit coach Lawrence Frank, a Celtics assistant last season. "The beauty of Boston is everyone does their part. They all chip in. It's never really a guy going for 35. They all kind of do their part. They were coming in with a sense of urgency."
Boston opened an 11-game stretch in which nine are at home.
The Celtics haven't started a season 0-4 since 1969-70 and were 0-3 for the first time since 2006-07.
"I don't think we played particularly well in those three games, but one thing I know is we know how to play basketball," Allen said. "We like who we have on our team. We knew we had to work harder."
The Celtics' defense had been poor until Friday night, when they held the Pistons to 43 percent shooting. And their offense was on target, as they hit 52 percent of their field-goal attempts and all 19 free throws.
"We lost a little bit of our poise on the offensive end, we were slower to get into stuff, plus they turned it up defensively," Frank said.
Allen opened the third quarter with a 3-pointer and Pierce hit two more in the first four minutes of the period, building the lead to 61-47. Bass ended the quarter with a jumper from the top of the key at the buzzer.
"That quarter has been giving us problems in all three games," Monroe said. "We've got to come out of halftime with more energy."
Detroit made a brief run, outscoring Boston 9-2 to close the gap to 90-76 with 6:24 left in the game. Then Allen hit two free throws, Kevin Garnett sank a jumper and O'Neal connected from the top of the key for the Celtics' last basket. It gave them a 96-76 lead with 3:57 to go.
The Celtics had gone ahead to stay on Allen's 20-footer that made it 17-15 with 4:24 left in the first quarter. They took their biggest lead of the half, 38-26, on two free throws by Pierce with 6:22 remaining.
"He gives us a swagger that was definitely missing the first three games," Celtics guard Keyon Dooling said of Pierce.
Pistons starting guard Rodney Stuckey made just one of 11 shots and finished with three points and seven assists. ... New Boston Red Sox manager Bobby Valentine watched from a front-row, baseline seat. Patriots wide receivers Deion Branch and Julian Edelman, safety Patrick Chung and running back Stevan Ridley also were in the crowd. ... Celtics backup forward Chris Wilcox missed his second straight game with a bruised left shoulder. Rivers said he could return on Sunday at the Washington Wizards.