Riley O’Brien has become one of the Cardinals’ most productive relievers through the first month and change of the 2026 season, a late-career rise that looked unlikely for years as he bounced from rotation prospect to bullpen arm to Triple-A depth. The 31-year-old right-hander entered play on May 7 with a path shaped by injuries to the sport’s calendar, multiple organizations and a long fight to harness his stuff.
The Cardinals have been one of the bigger surprises in the National League, and O’Brien has been a big reason why. He threw 48 of his 67.1 total innings in the majors last season and finished with a 2.06 ERA, numbers that read like a breakout even as his xERA, FIP and xFIP pointed toward regression. For a pitcher whose career has long been tied to command questions, that kind of run matters because it has given St. Louis reliable outs in high-leverage spots while the club keeps pushing into contention.
O’Brien’s climb began when the Rays took him out of the College of Idaho in the eighth round of the 2017 MLB Draft, and he opened his pro career as a starter. He put up a 2.75 ERA across 88 innings between both A-ball levels in 2018 and followed with a 3.05 ERA over 102 innings between High-A and Double-A in 2019. Then the 2020 minor league season was wiped out by the pandemic, and after that lost year he was traded to the Reds.
By 2021, O’Brien had logged 112 innings with a 4.55 ERA and reached the majors with Cincinnati. Seattle moved him to the bullpen in 2022, but he threw only one inning in the majors and 39 innings in Triple-A that season. He spent all of 2023 in Triple-A, then moved on to St. Louis in 2024, where he spent most of the year back in the minors before getting eight innings at the big-league level.
What has changed is less the jersey than the shape of his game. O’Brien ascended through the minors as a cutter-sweeper dominant arm, but he has evolved into more of a sinker-sweeper-slider reliever, and his walk rate had bottomed out at 6.81 BB/9 by 2022. That revamped arsenal has helped push the 2022 version of him further into the background. The next test is whether he can keep missing bats and limiting damage as the Cardinals try to stay among the National League’s early surprises.