riley o'brien has turned into one of the St. Louis Cardinals’ most important bullpen arms at age 31, and the first month and change of 2026 has put his name alongside the club’s most productive relievers. He entered the season with little national attention, but his work has helped steady a Cardinals staff that has been one of the bigger surprises in the National League.
O'Brien’s 2026 surge
O’Brien’s production has been the clearest change. Prior to play on May 7, he had become one of the most productive relievers in baseball, a jump that fits the Cardinals’ need for reliable late-inning innings as the season has taken shape.
His current run is built on a different mix than the one that carried him through the minors. O’Brien has evolved from a cutter-sweeper dominant arm into more of a sinker-sweeper-slider reliever, a shift that has gone with his rise in St. Louis. The change has come after years in which strike-throwing kept him from settling into a permanent major league role.
From College of Idaho to St. Louis
The path here took a long time. The Tampa Bay Rays drafted O’Brien out of the College of Idaho in the eighth round of the 2017 MLB Draft, and he began his professional career as a starter. He posted a 2.75 ERA across 88 innings between both A-ball levels in 2018, then a 3.05 ERA across 102 innings between High-A and Double-A in 2019.
The 2020 minor league season was wiped out by the pandemic, and he was traded to the Reds after that. O’Brien threw 112 innings with a 4.55 ERA in 2021 and made his MLB debut with Cincinnati. He moved to the bullpen with Seattle in 2022, when he threw one inning in the majors and 39 innings in Triple-A, then spent all of 2023 in Triple-A.
Cardinals role and roster pressure
St. Louis gave him another opening in 2024. He spent most of that season back in Triple-A before logging eight innings in the majors, then followed with a larger role in 2025, when he threw 48 of his 67.1 total innings at the big-league level for the Cardinals and posted a 2.06 ERA.
There is a complication in that 2025 surface line. His xERA, FIP and xFIP all pointed toward regression, so the Cardinals were not just buying a low ERA — they were betting that the underlying shape of his work would hold up. Through the first month and change of 2026, that bet has paid off in leverage innings, and it could leave St. Louis with a meaningful bullpen decision later in the summer if his role keeps expanding.
