O'Reilly, selected by Colorado in the second round of the 2009 Entry Draft at the Bell Centre in June, scored with 6:13 remaining to give the Avalanche a 3-2 victory over the Canadiens on Thursday night.
The 18-year-old, chosen with the 33rd pick by the Avalanche four months ago, got his own rebound and banged it past Carey Price for his first goal in seven games, giving the surprising Avs their third win in four nights, all on the road.
Second-round picks almost never make it to the NHL at age 18, and O'Reilly said he never expected to be playing at the Bell Centre in October after being drafted there in June.
''No, I never thought I'd be here this far, at least,'' said O'Reilly, who said he plans to give the milestone puck to his grandmother. ''It's pretty amazing. You never know when it's going to happen and finally it does and you're like, 'Wow!' This is what you dream about forever. And after I scored I was like, 'What do I do now?' I didn't know what to do, how to celebrate or anything, I was feeling so good.''
Craig Anderson stopped 29 shots to improve to 5-1-1 -- he's played every minute this season for the Avs, who look nothing like the team that finished last in the Western Conference in 2008-09. Anderson survived a furious scramble by the Canadiens in the final seconds after Montreal pulled Price for an extra attacker.
''It was exciting,'' said Anderson, who signed with the Avs during the summer after playing in Florida. ''The fans were excited, I was excited -- I got a lucky break. I think I made one save that was kind of a scramble in front, it went to their guy and he kind of shoveled it to the net and it hit my toe and went right back to him. He had a wide open net and chipped it over my leg and I wonder how he missed the net. We've just got a little luck on our side right now and we'll take it. We got two points and let's get out of here.''
Colorado got second-period goals from Milan Hedjuk and Kyle Cumiskey -- both on bank shots off Montreal defensemen -- to improve to 3-1-1 on its seven-game trip.
''We're a pretty resilient group right now,'' Avalanche coach Joe Sacco said. ''We knew they were going to come out with lots of energy. Unfortunately we had some penalties and they started to get some chances but we responded very well in the second period. We came out and that may be one of the best periods we've had all season, and it's just good to see our guys respond like that. We battled hard and we've got good compete in our game.''
Roman Hamrlik and Tomas Plekanec scored for Montreal, which has lost four in a row since starting the season with back-to-back overtime wins. Hamrlik scored the only goal of the first period, and Plekanec tied it at 2-2 when he banged Andrei Kostitsyn's pass through Anderson's pads at 8:26 of the third.
The Canadiens dominated the opening period but were outplayed after that, spoiling the milestone opener. They are 56-27-16-1 overall in home openers, including two wins and six losses during their first eight seasons in the National Hockey Association before they joined the NHL in 1917-18 as one of the League’s founding franchises.
"I don't think we're doing enough to win games," said newcomer Mike Cammalleri, a free-agent signee who is still without a goal as a Canadien. "I don't think we should leave things to chance.
"You don't want to win because you could have won, you want to win because you definitely should have won."
Hedjuk tied the game at 13:39 of the middle period when his shot hit Josh Gorges' skate and skidded between Price's pads. Cumiskey put Colorado ahead with 28 seconds left when his shot from the left wing hit rookie Shawn Belle and deflected into the net.
'''We were back on our heels in the second period and they were taking it to us and that's what happens,'' Canadiens forward Brian Gionta said. ''It doesn't matter what the breaks are, we let up in that second and that's what caused it."
--John Kreiser, NHL.com
Material from wire services and team media was used in this report
1 - 0 MTL
1 - 1 Tie
2 - 1 COL
2 - 2 Tie
3 - 2 COL
Too many men/ice - bench