Cardinals' next great bullpen arm is blossoming before our very eyes

The St. Louis Cardinals are coming off two great series wins against playoff contenders and look to keep the momentum going against the lowly Colorado Rockies. After taking four games against the Dodgers and Cubs, the Cardinals could play their way back into the postseason picture.

St. Louis appeared to punt on the season at the trade deadline when they shipped three expiring reliever contracts for young minor league pieces. A tough July made this the correct choice, but the Cardinals have turned around in August and go into play against the Rockies just 3.5 games out of the final Wild Card spot. Even with those new vacancies in the bullpen, the Cardinals have pulled arms up from Memphis, and one guy in particular has been effective in his new role. Riley O'Brien, whom the Cardinals received in an offseason trade with the Seattle Mariners in 2023, has stayed healthy and finally showcased the skillset that made him an eighth-round pick in the 2017 draft.

Cardinals reliever Riley O'Brien has become one of the team's trusted late-game arms out the bullpen.

After the Cardinals traded Ryan Helsley, Phil Maton, and Steven Matz, the biggest questions were who would take those relief innings and whether they would be as effective as the previous pitchers. One of these options was O'Brien, whom I have been extremely vocal about getting a chance since the Cardinals brought him over from Seattle, but injuries and command issues have limited his big league innings. Coming into this year, O'Brien had appeared in 10 major league games since 2021 and averaged more than a walk per inning to go along with his electric strikeout stuff.

However, the 2025 season has been a different story for the reliever despite a tough start to the campaign. After being optioned three times so far this year, O'Brien has been on the team since June 9 and has taken off since being covered by colleague J.T. Buchheit. Since his recall, O'Brien has appeared in 22 games, covering 26.2 innings and most recently notched his first career save in a win against the Dodgers after picking up a win just two days earlier. Over those games, the reliever has a 1.01 ERA and 27 strikeouts against 13 walks.

As with most relievers, O'Brien is doing this mainly with a two-pitch mix: a 98.3 mph sinker and 90 mph slider. He throws those two pitches 80% of the time and has a curveball, which he has started to mix in more frequently, with great results. Opposing batters are hitting just .193 against O'Brien, and he has only allowed two homers all season and zero since his first game after his recall. Also encouraging is the fact that he has performed his best late in the game. In the eighth inning or later, he has thrown 13.2 innings, striking out 10 and allowing just two runs. His command remains an issue at times, but he has shown improvement, only walking three batters over his last 6.2 innings, covering a span of seven appearances.

While manager Oliver Marmol has yet to commit to one closer for the last push of the season, Riley O'Brien has to be near the top of the list for guys who can fill that role. JoJo Romero seemed likely to get some chances, but he is the only lefty in the bullpen, so he is still used in high-leverage matchup opportunities. If O'Brien can keep his momentum going through August, there's little reason to believe he won't become a favorite for the closer role in September and beyond.