Charlie O'Brien joined Cardiff City FC in July 2025 from Cobh Ramblers.

A committed defender, O'Brien was one of four additions made at U21 level in the summer of 2025, joining alongside Jack Kingdon, Gabriel Keita and Matty Apter.

His first taste of senior action at City was to gain experience training with the First Team during the pre-season tour of Spain before the 2025/26 season.