Having been named in Ireland's squad for the 2015 Under-20's Six Nations Championship on 21 January 2015, Oliver made his debut for Ireland U20 on 13 March 2015, starting for the side in their 19–12 defeat against Wales U20 at the Eirias Stadium, Colwyn Bay. On 25 May 2015, Oliver was selected in the Ireland Under-20 squad for the 2015 World Rugby Under 20 Championship. On 6 June 2015, he started against Scotland U20 in a Pool C fixture, scoring a try in Ireland's 24–20 win, but also getting sin-binned during the game. On 15 June 2015, Oliver started in the 5th-8th place play-off against Wales U20, which Ireland lost 22–12.