Sammy Blais scored, and Jordan Binnington made 29 saves for the Blues (26-26-3), whose three-game winning streak ended.
"Yeah, it's one of the tougher trades I've definitely dealt with," St. Louis forward Brayden Schenn said. "I've known [O'Reilly] since I've been 17 and obviously played some Canada events and many good years here. He's a guy that comes to the rink and lots of laughing and joking with him. On the flip side of that, he demands a lot of out of his teammates and works extremely hard and pulls guys into the fight each and every night. Guys like that are obviously great to play with.
"Shocking seeing the news about both of those guys going early, two character guys that ... I guess shocking when you saw it, but I guess it's been talked about for so long, it doesn't really come to a shock with two weeks before the (2023 NHL Trade Deadline on March 3). Obviously two great team guys and Toronto got two guys that know what it takes to win."
Rantanen's first goal in six games was also his 200th in the NHL, a one-timer from the right face-off circle that gave Colorado a 1-0 lead at 16:37 of the first period.
Byram made it 2-0 at 7:37 of the second period, a shot from the left circle that rode up and off the stick of Blues defenseman Colton Parayko.
"It's part of what our identity is," Colorado coach Jared Bednar said. "Our [defensemen] have always been active participants in our offense. 'Bo's' obviously missed some time, but getting back up to speed, we're starting to see him contribute a little bit more."