Oisín, who comes from Clifden Galway (in the west of Ireland), started playing rugby from the age of 7 at his local rugby club “Connemara Blacks” where he showed a love for the game. He attended a Boarding School in Ireland that had a reputation for Rugby and Hurling called Cistercian College Roscrea in Leinster.
During this period, he was selected within the Leinster Rugby Schools set up which gave him an insight into a professional team at a young age and fuelled his desire to become a professional rugby player. Upon leaving school, he progressed and became a member of the Leinster Rugby Academy. While in the Academy, he was selected to represent the Ireland U20s for two seasons in the U20 Six Nations and Junior World Cup making 20 caps over that period for his Country. During this period within the Academy he also made 3 senior appearances in the Pro14 for Leinster Rugby.
He spent a stint in Australia for 8 weeks playing in the Queensland Premier Rugby with Bond University in the Gold coast which gave him a taste of a different style of playing rugby that has helped him develop as a player.