on july 3, 2018, o'brien signed a contract with the toronto marlies of the ahl. he attended the marlies training camp but was assigned to their echl affiliate, the newfoundland growlers. prior to the 2018–19 season, the growlers inaugural season, o'brien was named an assistant captain. while playing for the growlers, o'brien scored the first hat trick in franchise history to help beat the reading royals 5–4 on november 16, 2018. he scored his second hat trick with the growlers in a 5–1 win over the jacksonville icemen on january 5, 2019. on february 25, o'brien was recalled to the ahl along with teammates hudson elynuik, brady ferguson, and josh kestner. o'brien was reassigned to the echl along with kestner on february 28 where he then helped lead the growlers to the 2019 kelly cup playoffs. he was subsequently awarded the june m. kelly award as mvp on june 5, after winning the cup.