It was the first time in Blues history they won in regulation after trailing with one minute remaining, and the 11th time in NHL history.
"The first two periods I thought there wasn't a ton of energy," coach Craig Berube said. "We weren't winning enough puck battles. I thought they did a good job of clogging things up, being tight on us, not giving us any room. But we didn't want to compete through that. We kind of allowed it to happen more than anything.
"We needed to get on our toes and start skating and start winning puck battles and doing things right. Guys did a good job responding. I thought we had a real good third period going. We took the penalty late, killed it off. Special teams were really good tonight, [and] that's the difference in the game."
Jason Robertson scored, and Braden Holtby made 29 saves for the Stars (17-13-2), who had their four-game winning streak end. Dallas has lost its past five on the road, including two straight to St. Louis.
"It definitely wasn't the result we wanted, but I think [Stars coach Rick Bowness] told us that was almost a perfect road game for us," Robertson said. "We had a bunch of chances and we tried to limit their chances. It's just hockey at the end of the game."
O'Reilly tied it 1-1 with 47 seconds remaining when he scored on a rebound in the slot. After the Blues pulled Binnington for a 6-on-4 advantage, Torey Krug's shot caromed off the end boards, past St. Louis forward Brayden Schenn to O'Reilly for the goal past a sprawled-out Holtby.
"It was mayhem. … It was a little bit chaotic," O'Reilly said. "But I thought we had a good third period. We had some good chances, and it took us right down to the wire to tie the game up, but yeah, there was pucks and bodies everywhere and obviously, we get a good bounce and in the back of the net to tie it up. Yeah, it was good."