O’Brien began the 2025 season with the Tri-City ValleyCats, where he appeared in seven games over 9.1 innings pitched and posted a 6.75 ERA before being traded to the Bird Dawgs in exchange for a 2026 first-round draft pick. With Down East, he made 16 appearances, including two starts.

Prior to the 2025 season, O’Brien spent time with both the Joliet Slammers and Tri-City ValleyCats in 2023. He made 18 appearances with Joliet, posting a 5.24 ERA, before joining the ValleyCats and making 12 appearances with a 5.06 ERA. He missed the 2024 season due to injury. O’Brien made his professional debut in 2022 with Joliet, appearing in 14 games and recording a 5.14 ERA.

O’Brien played five collegiate seasons at the University of North Carolina, where he finished his career ranked fifth in program history with 112 appearances. He compiled a 14-3 record with 10 saves and a 3.91 ERA. As a freshman, he became the ninth pitcher in program history to win at least seven decisions without a loss in a single season and earned Second Team Freshman All-America honors from D1Baseball.

A native of Maiden, North Carolina, O’Brien attended Maiden High School, where he was ranked the No. 5 left-handed pitcher in the state and threw three no-hitters during his prep career.