5 Things to Know About Pitching Phenom Riley O'Brien Ahead of His MLB Breakout

St. Louis — St. Louis Cardinals closer Riley O’Brien is turning heads across Major League Baseball this week as he returns to his Pacific Northwest roots for a pivotal interleague series against the Seattle Mariners, the club that once drafted him and the city where he first learned the game. A RESILIENT ROAD BACK The 30-year-old right-hander opened the 2025 season on the injured list with shoulder soreness, but since being activated in late July he has been lights-out: a 0.93 ERA, 0.88 WHIP and 28 strikeouts across 19⅓ innings. His most recent outing — a clean, one-strikeout eighth inning that earned him the win against San Francisco — pushed his record to 3-0 and underscored why manager Oli Marmol now trusts him in the ninth. WHY THE CARDINALS NEEDED A NEW CLOSER St. Louis’ bullpen has battled inconsistency all year, and injuries to Giovanny Gallegos and Ryan Helsley left a vacuum in the late innings. O’Brien’s four-seam fastball, which averages 97 mph, pairs with a sweeping slider that hitters are whiffing at a 46 percent clip. Since August 1, opponents are batting just .131 against him — the best mark among National League relievers with at least 15 innings pitched. HOME-FIELD NARRATIVE BOOSTS MOMENTUM For O’Brien, the Seattle matchup is more than a statistical footnote; it’s a personal milestone. He grew up in Shoreline, Washington, starred at Shorewood High, and made countless childhood trips to T-Mobile Park. “I’ll have 30-plus family members in the stands,” he told reporters. “That adrenaline is going to be real, but my job is the same: fill up the zone and get three outs.” The Cardinals enter the series just two games back in the NL Central, and every save opportunity now carries playoff implications. STAT LINE THAT POPS • 2025 Season (through Sept. 10): 3-0, 11 SV, 0.93 ERA, 38 K, 3 BB in 29 IP • First-pitch strike rate: 71 % (MLB average: 63 %) • Hard-hit percentage allowed: 20.4 % (Top 5 among relievers) Up-to-date statistics via ESPN game logs. FANTASY & BETTING ANGLE With Helsley still weeks away, O’Brien’s closer job looks secure. Fantasy managers scrambling for late-season saves should prioritize him immediately; his strikeout upside also offers a boost in ratio categories. Sportsbooks have taken notice, moving his odds to record a save in any given Cardinals win from +180 in mid-August to +105 this week. WHAT’S NEXT If O’Brien can shut the door in Seattle, national chatter around his breakout will only intensify. The Cardinals travel next to Milwaukee, where division-leading Brewers hitters have the league’s fourth-highest strikeout rate versus sliders — yet another favorable matchup for the surging right-hander. BOTTOM LINE Riley O’Brien’s homecoming arrives at the perfect time for both player and club. His dominant run since returning from injury has stabilized the Cardinals’ bullpen, vaulted him into fantasy relevance, and positioned him as one of baseball’s feel-good comeback stories of 2025. Expect the buzz to keep building — and for O’Brien to keep pounding the strike zone when it matters most.