Boston's Baumgartner, who played all 82 regular season and six playoff games last year without scoring a goal, scored 3:17 into the third period as the Bruins and Blues tied 3-3 Saturday night.
For a while, it looked the the Blues' offense would be enough to beat Boston in its initial post-Brett Hull era contest. But a fluke play spoiled St. Louis' night.
Hull, who signed with Dallas as a free agent during the offseason, left with a club all-time high 527 goals and led the team in goals nine of the past 10 years.
"There's two ways to look at it," St. Louis defenseman Al MacInnis said. "It's good to get a point, but we had the lead."
The Bruins extended the NHL's current longest opening-game unbeaten streak to 10-0-2, with their last loss a 4-2 setback against Calgary on Oct. 9, 1986.
"He's synonmous with St. Louis," said Boston's goaltender Byron Dafoe of not facing Hull. "As a goaltender, its kind of nice. But I fear he's still in Dallas. Brett Hull or no Brett Hull, though, they're still a good hockey club."
Pavol Demitra scored a pair of goals and Tony Twist added the other one for St. Louis. Steve Heinze and Sergei Samsonov had power-play goals for Boston, which started its 75th season.
With St. Louis holding a 3-2 lead, goaltender Grant Fuhr came out and mishandled a loose puck that Baumgartner tipped away before sliding into the net, tying the game at 3. It was only his 13th goal in 710 regular-season games, and his first with the Bruins.
"I was trying to press the defense to make a pass before they wanted to," Baumgartner said. "I was able to put a backhander on net. I really didn't see it go into the net until my teammates came over to me. I hope its not quite so long before I get another one."
His last goal came when he was with Toronto on Jan. 17, 1996.
The best chance in overtime came with 32 seconds left when Fuhr came out to challenge Rob DiMaio, who fired a wrist shot from the left wing, for a pad save.
Twist tipped MacInnis' shot past Dafoe 8:59 into the first to give the Blues a 1-0 lead. St. Louis made it 2-0 on the power play 7:08 later when Demitra redirected MacInnis' shot inside the left post.
The Bruins cut it to 2-1 when Heinze slid a shot past Fuhr from in close after Samsonov backhanded a pass from the side of the net.
Demitra scored on a breakaway 13:31 into the second period, making it 3-1. He broke in alone, shifted Dafoe out of position and tucked a backhander in after taking a pass at the blue line from Pierre Turgeon.
Samsonov's score cut the margin to 3-2 with 3:52 left in the second period.
The Bruins honored last season's coach of the year, Pat Burns, and Caler Trophy winner, Samsonov, in a 15-minute pregame ceremony. Three of Boston's former rookie of the year winners, Bobby Orr, Derek Sanderson and current defenseman Ray Bourque presented the trophy to Samsonov, with Orr receiving a lengthy ovation.
Boston was without a pair of unsigned regulars from last season - forwards Anson Carter and defenseman Kyle McLaren.
"The opening of the game we seemed to have some jitters," Burns said. "We found a way to tie it and get a point."
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