"With the trade, it's disappointing to see that I wasn't a guy or a piece that could help, but I'm more excited that there's another team that wants me," he said. "I get a chance to go to a great city and a great team and a team that's got a ton of good pieces and they are trying to win right now and that's something I really want to be a part of."
The 27-year-old center had 61 points (24 goals, 37 assists) in 81 games with the Buffalo Sabres this season and was second in the NHL in face-off winning percentage (60 percent).
[RELATED: 2018-19 NHL Trade Tracker]
"I'd been working with Buffalo for some time on a potential trade," St. Louis general manager Doug Armstrong said. "It fell apart a little this morning and we were able to pick it up after that and add and subtract some pieces that made everyone comfortable in the end."
Buffalo receives forwards Patrik Berglund, Vladimir Sobotka and Tage Thompson, a first-round pick in the 2019 NHL Draft, and a second-round pick in the 2021 NHL Draft.
"Buffalo did a nice job of maintaining their desire for Tage and the picks," Armstrong said. "Then you have to do the match to make it work. We had to part with some players we respect highly. But in a [salary] cap system, you have to do things that are uncomfortable."
O'Reilly has five seasons remaining on a seven-year contract that runs through the 2022-23 season.
"We'll certainly miss Ryan's productivity because he is a top-end player for us," Sabres general manager Jason Botterill told The Buffalo News. "But that's why it was key to get NHL players back. ... We felt as a management group at the end of the season we had to change the dynamic."
The Blues paid a signing bonus O'Reilly was owed July 1, the newspaper reported. The Sabres would have had to pay it if O'Reilly was traded after that.
"If it went past midnight today, the asking price was certainly going to be a lot different," Botterill said. "You never say never, but it certainly played a role in making sure the deal was done today."
Selected by the Colorado Avalanche in the second round (No. 33) of the 2009 NHL Draft, O'Reilly was acquired by the Sabres with forward Jamie McGinn in a trade from the Avalanche on June 26, 2015, for forwards Mikhail Grigorenko, J.T. Compher, defenseman Nikita Zadorov and a draft pick.
O'Reilly has 422 points (155 goals, 267 assists) in 651 regular-season games and seven points (three goals, four assists) in 13 games in the Stanley Cup Playoffs.
Over the past three seasons, O'Reilly has won 58.3 percent of his face-offs, second in the NHL behind Antoine Vermette (59.2 percent for the Anaheim Ducks and Arizona Coyotes). Vermette led the NHL this season at 60.00, ahead of O'Reilly at 59.98.
The Blues earlier Sunday signed center Tyler Bozak to a three-year contract worth $5 million annually.
"I think that depth down the middle is very important," Armstrong said. "When you look at the strength at center in our (Central) Division, when you come with [Brayden] Schenn, O'Reilly and Bozak, (coach) Mike [Yeo] can play them against any line at any time. It was worth the risk."
Berglund, a 30-year-old center, had 26 points (17 goals, nine assists) in 57 games last season, when he did not debut until Nov. 29 after offseason shoulder surgery. He has 322 points (168 goals, 154 points) in 694 NHL games.
Sobotka, a left wing who turns 31 on Monday, had 31 points (11 goals, 20 assists) last season, his first full season with St. Louis since returning to North America following three seasons in the Kontinental Hockey League. He has 155 points (47 goals, 108 assists) in 463 NHL games with the Blues and Boston Bruins.
Thompson, a 20-year-old center, was the No. 26 pick in the 2016 NHL Draft. He had nine points (three goals, six assists) in 41 games this season, his first in the NHL.
"We're looking for more scoring from our wings," Botterill said. "What we see with Tage is a kid with a great shot, great size, has a really good reach on the ice, played NHL games as a 20-year-old, had an opportunity to win a World Junior gold medal (with the United States in 2017). We think he's a player that's going to be able to step in our lineup and grow with our young centermen."
O'Reilly said after the season that he "lost the love of the game multiple times" because the Sabres were losing.
"I had some great conversations with Ryan at the end of the season," Botterill said. "I knew if Ryan was coming back, he was certainly going to be a big part of changing things here in Buffalo. We just felt comfortable at this time with what St. Louis was offering."
NHL.com staff writer Tracey Myers contributed to this report