There’s a certain calm that settles over Busch Stadium when the bottom of the ninth inning belongs to the right arm. The tension doesn’t disappear, but it manifests itself in the belief that the game is about to end in your favor. For the Cardinals, that feeling has started to return in the form of closer Riley O'Brien.
In a season when the bullpen has been up and down depending on the day, O’Brien has been as steady as they come, sporting a 0.00 ERA in his first 10 appearances. Following years of Ryan Helsley shutting the door for the Cardinals through thick and thin, O’Brien is toeing the rubber with the dependability that teams dream of having.
Acquired from the Seattle Mariners back in November 2023 for cash considerations, O’Brien spent the majority of the 2024 season on the injured list, but he established himself as a high-leverage option for manager Oliver Marmol during the 2025 campaign, posting a 2.06 ERA in 42 appearances out of the bullpen.
The Cardinals emptied their stable of arms at the 2025 Trade Deadline, sending Helsley along with veterans Phil Maton and Steven Matz to contending teams, placing the fate of their late-game situations into the hands of many inexperienced arms, and O’Brien has risen to the occasion.
Since the Cardinals created that vacuum for O’Brien to step into, he’s posted a 1.72 ERA in 30 games between the final two months of the ‘25 season and the start of the ‘26 campaign.
O’Brien has seen a substantial uptick in the swing and misses he produces this year, raising his strikeout percentage to 31.4%, up from 22.6% in 2025, and now has well above-average chase percentage (34.2%) and whiff percentage (29.2%) to pair with his 100th percentile 73.9 ground-ball percentage.
How O’Brien is achieving those results is hardly a mystery. The right-hander has upped his sinker usage by 11% in 2026, throwing the pitch 70% of the time to right-handed hitters and 53% of the time to lefties. He has also opted to make his sweeper his primary breaking ball again over his slider, throwing the latter 24% of the time this year compared with 16% for the slider.
The movement profile on his sweeper is as good as it has ever been, averaging 17.1 inches of break and a 3.4 induced vertical break (defined as the amount of movement a pitcher generates from how he spins the baseball), allowing the pitch to tunnel really well with his sinker profile.
That sinker is the bread and butter of O’Brien’s game right now, with only Padres right-hander David Morgan (93.3%) producing more ground balls off that pitch than O’Brien (84.2%) in at least 15 batted ball events this year. What O’Brien does better than almost any reliever in the sport right now is produce elite whiff percentage on that same pitch (28.2%), sixth best in baseball. That combination of swing and miss and elite ground-ball output has allowed O’Brien’s sinker to produce a .175 xwOBA this season, the best of any sinker in the sport.
Missing bats and allowing soft contact when they do get a piece of something is always going to be the best way to get batters out, and the 31-year-old has seemingly mastered the latter while making the former a strength in 2026. That is how he’s gone from a reliever with electric stuff but average production to one of the most dependable closers in the sport.
Something to monitor for O’Brien moving forward will be his command, as he has struggled with it throughout his career, but he has yet to issue a single walk in ‘26. If he is able to limit free passes at even an average to above-average clip, his ability to pitch like a top bullpen arm in the sport feels more sustainable.
Paired with JoJo Romero at the back end of games, who is tied with O’Brien and two other relievers for the second-most scoreless innings in baseball this year, the Cardinals have two arms that can truly shut down games when they matter most.
For a club already developing a reputation as the “Cardiac Cardinals,” it’s nice to have your blood pressure drop when O’Brien takes the mound.