Born in Irun, Gipuzkoa, Basque Country, Oier arrived in FC Barcelona's youth academy at the age of 18, from local Real Unión.

[5] On 29 January 2017, Oier was loaned to Levante UD for the rest of the campaign with the option to buy in event of promotion; he replaced Álex Remiro, whose own temporary switch from Athletic Bilbao had been curtailed.

[6] Manager Juan Ramón López Muñiz began playing him in place of Raúl Fernández in the final matches,[7] including in the goalless draw at CD Tenerife on 20 May that saw the Granotas crowned Segunda División champions.

[9] Having only been used in cup games due to Paco López' preference for Aitor Fernández, Oier left for RCD Espanyol on 31 January 2020 for a €1.5 million fee, signing until June 2022 with the option of another season after that.

[14] On 30 December 2016, Oier made his one appearance for the unofficial Basque Country team, playing the second half of a 3–1 win over Tunisia at the San Mamés Stadium.