O'Neill's one-timer on the power play gave Toronto a 3-2 lead at 7:34 of the third period. He then pointed to the sky in tribute to his older brother, Donny, who was killed in a car accident in Toronto in July.
"The last three months have seemed like three years. I knew that when I got my first one it was for him," O'Neill said. "It was pretty emotional. It's even tough talking about it right now. It's something that is real tough to deal with, but being in this locker room and playing hockey makes it a little easier."
O'Neill was traded to Toronto from Carolina two weeks after his brother was killed. O'Neill was in Toronto at the time of the crash to attend a meeting in which the NHL players' association ratified the new collective bargaining agreement with the league.
Lindros scored with 6:51 left to make it 4-2 for the Maple Leafs (1-1-2). Lindros, a Toronto native, has four points in four games with his new team.
He spent his first nine NHL seasons with Philadelphia and was the league MVP in 1995. A bitter dispute with general manager Bob Clarke led to Lindros being traded to the New York Rangers in the summer of 2001. Lindros signed with the Maple Leafs after the lockout.
Philadelphia coach Ken Hitchcock pulled goalie Robert Esche in favor of an extra attacker with 1:18 left, but the Flyers couldn't cut into their deficit.
Backup Mikael Tellqvist started in goal for Toronto in the second of back-to-back contests. He made 31 saves one night after the Maple Leafs lost in a shootout at Ottawa.
"Hopefully, I can ride this wave a little and get a chance to play again," said Tellqvist, who has struggled with the Maple Leafs in the past.
Gagne said he was lucky to score.
"He played very well. He made some big saves, especially in the second period," Gagne said. "I don't want to say we took the game over, but we played better in the second. We had a couple of great chances and he made big saves."
Darcy Tucker gave Toronto a 1-0 lead at 7:28 of the first after Esche couldn't clear a rebound
Knuble tied it with a short-handed goal at 17:47 after a Leafs defenseman failed to check him in front of the net.
Gagne tied it at 2 with just over two minutes left in the second period by putting a rebound past Tellqvist, who was facing a barrage of shots.
- The start of the second period was delayed for 30 minutes after the lights didn't fully come on. Both teams returned to the dressing rooms after skating around the ice.
- The Maple Leafs honored longtime Blue Jays broadcaster Tom Cheek, who died Sunday after a battle with brain cancer. His most famous call -- Joe Carter's 1993 World Series winning home run -- was shown on the video board.
- Flyers D Chris Therien missed the game because of back spasms.