Every season sees some unlikely bullpen breakouts, and Riley O’Brien fits that bill. He’s not a prospect, nor a former top-100 name finally making his mark. O’Brien has, however, become one of the most productive relievers in baseball during the 2026 season.
For a Cardinals team that has been one of the bigger surprises in the National League, O’Brien’s emergence has been a stabilizing force. It could become one of the more fascinating roster decisions of the summer, depending on where the Cardinals are in July.
O’Brien’s Path to the Cardinals
O’Brien was drafted out of the College of Idaho in the eighth round of the 2017 MLB Draft by the Tampa Bay Rays. He started his professional career as a starter and quickly showed promise, posting a 2.75 ERA across 88 innings between both A-ball levels in 2018. He followed it up with 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 O’Brien was eventually traded to the Reds. In 2021, he threw 112 innings with a 4.55 ERA and made his MLB debut with Cincinnati, but his path to a stable big-league role remained unclear. By 2022, he had transitioned to the bullpen with Seattle, throwing just one inning in the majors and 39 innings in Triple-A. He spent all of 2023 in Triple-A, then moved on to St. Louis, where he spent most of 2024 back in Triple-A before logging eight innings at the major league level.
The 2025 season was the first time O’Brien really carved out extended MLB opportunity. He threw 48 of his 67.1 total innings at the big-league level for the Cardinals and posted a 2.06 ERA.
Command Key to O’Brien’s Development
O’Brien ascended through the minors as a cutter-sweeper dominant arm with legitimate bat-missing traits, but his lack of strike-throwing consistently kept him from settling into a permanent MLB role.
After hovering around a 4.00 BB/9 during his first two professional seasons, the command backed up considerably. By 2022, he had bottomed out with a 6.81 BB/9, the kind of walk rate that can hinder a player’s progress.
At 31 years old, O’Brien is not a prospect, and did not enter the season with much national attention. But through the first month and change of the 2026 season, the St. Louis Cardinals reliever has turned himself into one of the most important arms in St. Louis.