O'Brien embraces closer's mindset in enlightening rookie season

This story was excerpted from John Denton's Cardinals Beat newsletter, with writer/researcher Theo DeRosa pinch-hitting for this installment. To read the full newsletter, click here. And subscribe to get it regularly in your inbox.

SAN FRANCISCO -- Riley O’Brien slapped his right palm across the Cardinals logo on his gray road jersey.

The rookie reliever was still buzzing with elation on his way off the mound, striding toward home plate to shake hands with catcher Jimmy Crooks and retrieve the baseball. O’Brien had just come through in the clutch during the Cardinals’ Sept. 23 game at Oracle Park, entering with a one-run lead and a runner on second with two outs in the ninth and striking out the Giants’ Wilmer Flores swinging to end the game.

Yet again in 2025, O’Brien had shown a closer’s mentality that belied his limited MLB experience.

“You can see it in his face,” Cardinals reliever John King told MLB.com. “You just see the confidence out there and the willingness to attack.”

Those characteristics typified O’Brien’s rookie season -- his fourth season in the Major Leagues, officially, but the right-hander still retained rookie eligibility through the end of 2025.

O’Brien finished an excellent year with a 2.06 ERA in 48 innings, striking out 45. He picked up six saves in nine chances for the Cardinals, largely taking over the closer’s role after Ryan Helsley was dealt to the Mets at the 2025 Trade Deadline.

In 19 of his 20 post-Deadline appearances, O’Brien entered the game in the eighth inning or later.

“That’s always your goal in the bullpen, to throw in those late-inning situations,” O’Brien told MLB.com in San Francisco. “I’ve really been enjoying it. I feel like it brings out my best stuff.”

The 30-year-old certainly has an impressive arsenal. O’Brien’s two most-used pitches are his 98.0 mph sinker and a slider that averages 90 mph, but his curveball is his best offering. Hitters could hardly touch it in 2025, going 2-for-34 (.059) with 19 strikeouts.

O’Brien’s curveball has elite horizontal movement, and added velocity has taken the pitch to the next level. At an average of 83.4 mph in 2025, O’Brien’s breaking ball comes in harder than most.

“It seems like once it ticked up a couple miles an hour, I definitely started having more success with it, a little more swing and miss,” O’Brien said.

In 2025, O’Brien’s fastball velocity was up over 1 mph from the prior season, partly due to health. The pitcher made the Cardinals’ Opening Day roster in 2024 but appeared just once -- striking out the Dodgers’ Shohei Ohtani and Teoscar Hernández -- before a right forearm flexor strain cost him more than four months. He finished the year with an 11.25 ERA in eight Major League innings.

“You look at where he was last year to this year, there’s been a lot of growth -- not only in his ability to execute but how he handles pressure and stressful innings,” Cardinals manager Oliver Marmol said. “When things don’t go his way, [he has] the ability to bounce back.”

Throughout his career, O’Brien has exemplified that last point. Drafted by the Rays in 2017, he was traded to the Reds in 2020 and made his Major League debut for Cincinnati on Sept. 28, 2021. He was traded again to the Mariners (for whom he pitched once, in 2022) then dealt to the Cardinals after the 2023 season.

After “dealing with some stuff” injury-wise throughout 2024, O’Brien was more than happy to have a full, healthy season in the Majors in 2025.

“I feel like there’s been a lot of highs and lows,” he said. “This season has definitely felt like a high. I feel like things have started to click and I’m hitting my rhythm, so I’m feeling good right now.”

O’Brien is expected to be a key piece of the Cardinals' bullpen moving forward. After St. Louis traded away three relievers (Helsley, Steven Matz and Phil Maton) on expiring contracts, the rest of the club’s relievers are all arbitration eligible or, in O’Brien’s case, pre-arbitration.

At age 30, O’Brien hardly had a typical rookie season, but the maturity he showed is an excellent sign for the Cards.

“The growth that he’s had over the last year has been phenomenal,” King said. “The trust in himself is really apparent, and you’re seeing that. You’re seeing him grow and be more comfortable in those situations.”

When the pressure is on, O’Brien feels right at home.

His late-inning success has earned him plenty of self-assurance -- and the Cardinals’ confidence as well.

“It’s great to have them trust me,” he said. “Hopefully they continue throwing me in those spots.”