Riley Unroe

Riley O'Brien is a 31-year-old relief pitcher for the St. Louis Cardinals who has emerged as an unlikely bullpen breakout success story in 2026. After a lengthy minor league career with multiple organizations (Tampa Bay, Cincinnati, Seattle), O'Brien was acquired by the Cardinals in November 2023. Through early May 2026, he has established himself as one of the most productive relievers in baseball and a stabilizing force for a surprising Cardinals team. His success appears backed by tangible improvements in his K% and BB% rates, suggesting this is not merely a small-sample hot streak but a legitimate late-career breakthrough. At 31 years old, he represents a rare example of a non-prospect arm achieving elite-level performance.