The 22-year-old center is expected to make his season debut Sunday as the Avalanche kick off a three-game trek against the NHL-worst Columbus Blue Jackets.
O'Reilly, who led Colorado with 37 assists and 55 points last season, is set to take the ice for the first time following a lengthy holdout. The restricted free agent signed an offer sheet with Calgary before Colorado matched the two-year, $10 million deal.
"I'm just so excited now to be playing hockey and be back with these guys," O'Reilly said. "I can't believe how nice everyone has been about it and how great they've been to just accept me back -- it's like I haven't left. It just shows the character of this team.
"Hopefully I can contribute as best I can. It will probably take a little while to get back in the swing of things and catch up to the speed of the game."
The Avalanche (8-8-3) announced the signing during the first period of Thursday's thrilling 5-4 victory over the Flames. Colorado trailed 3-0 after the first 20 minutes and entered the third down 4-2 before storming back.
"That was great news," 20-year-old captain Gabriel Landeskog said of O'Reilly's signing. "I know there were 24 guys who were happy to have him back."
Landeskog had a goal and two assists while Matt Duchene recorded two points, including the go-ahead score 10:52 into the third.
"It was time to get back to what makes us a good team," Duchene told the club's official website. "We had a great second period. I think after we got those two goals we really believed that we were going to win the game. You could feel it in here. It's a great feeling right now. It's one of the best Avalanche wins in recent memory, that's for sure."
While Colorado improved to 6-2-1 at home, it's struggled in opposing arenas. The Avalanche have been outscored 37-22 in going 2-6-2 on the road, where they've posted a league-low 65.1 penalty-kill percentage.
Colorado, though, has taken 10 of 13 from the Blue Jackets (5-12-4), including a 4-0 win Jan. 24. Semyon Varlamov made 33 saves while Duchene scored twice and added an assist. The Avalanche, who are 7-2-1 when Duchene posts at least one point, have gone 5-1-0 in Columbus since 2009-10.
The Blue Jackets return home mired in a 1-5-2 stretch after suffering a 4-3 overtime loss at NHL-leading Chicago on Friday.
"Regardless of who it is, it's still a loss," coach Todd Richards said. "We still just aren't quite good enough. We battled and competed, which is what we should do, but we've got to find ways to come out on top."
"We did some pretty good things to be able to stick with those guys, but we're not doing enough to get that next goal," Steve Mason said after making 31 saves. "We knew it would be a real test for us because we had a lot of guys out."
The Blue Jackets could catch a break if Colorado goes with Varlamov, who is 0-6-1 with a with a 3.73 goals-against average over seven road starts. Varlamov, however, went 2-0-0 with one shutout and a 0.96 GAA at Nationwide Arena last season.