O'Neill was still soaking things in Friday afternoon, prior to the Cardinals' game against Cincinnati.
"It's a lot to take in," O'Neill said. "More excitement and anticipation than anything."
O'Neill was selected by Seattle in the third round (85th overall) of the 2013 Draft.
He joins a crowded outfield in St. Louis, but Cardinals manager Mike Matheny indicated that O'Neill will get plenty of at-bats.
"I've worked hard to get here, and now my job is to produce so I can stay here," O'Neill said.
• Matheny gave slumping outfielder Dexter Fowler the day off Friday when the Cardinals opened a three-game series at home against the Reds. Fowler is hitting. 176 with two homers and seven RBIs in 17 games, and he's 0-for-12 with six strikeouts in his last three games.
[We're] trying to help him get right," Matheny said. "Just want to get him to a better place. We're still trying to figure out what he needs."
Fowler is 1-for-17 against left-handers this season, and lefty Brandon Finnegan got the start for Cincinnati on Friday.
• Cardinals president of baseball operations John Mozeliak indicated that RHP Sam Tuivailala likely will head on a rehabilitation assignment sometime next week. Tuivailala was placed on the 10-day disabled list on April 12 due to a left knee strain.
• Cardinals RHP Alex Reyes is progressing nicely at extended spring training, according to Mozeliak.
"All signs are positive," Mozeliak said. "He's currently getting his work in."
Mozeliak indicated that Reyes likely will be sent on a rehab assignment sometime in the middle of May.
Reyes missed all of last season after undergoing Tommy John surgery on his right elbow in February 2017. He went 4-1 with a 1.57 ERA in 12 appearances for the Cardinals in 2016, with 52 strikeouts in 46 innings.
Steve Overbey is a contributor for MLB.com based in St. Louis.