A product of hometown CD Izarra's youth academy, he completed his development at neighbouring club CA Osasuna.

[5] In his first full professional campaign, Oier started the first five matches at right-back as starter César Azpilicueta was away on international duty.

[12] Oier extended his contract for three years in June 2015,[13] and in December 2018 this was lengthened to 2020 with a buyout clause of €8 million while the side remained in the second tier.

[16][17] On 31 October that year, in a 3–1 home loss to Atlético Madrid, he made his 300th official appearance for the team but also gave away a penalty by handball.

[18] Oier moved abroad for the first time at the age of 36, joining AEK Larnaca FC of the Cypriot First Division.