Oier made his debut for the first team on 2 January 2008, in the 2–2 home draw against CD Alcoyano in the round of 32 of the Copa del Rey.[2] More than one year after, on 17 May, he appeared for the first time in La Liga as Barça were already champions, in a 2–1 away loss to RCD Mallorca.[3]
For the 2009–10 campaign, after Albert Jorquera left for neighbours Girona FC, Oier was promoted to third choice. He still continued to appear regularly for the B's, however.
Granada
On 11 July 2014, Oier signed a three-year deal with Granada CF for an undisclosed fee.[4] On 25 August of the following year, he was loaned to fellow top-division side Real Sociedad in a season-long deal.[5]
Oier took the starting place from Fernández in the top flight, and signed a new deal in June 2018 to last until 2021, with a further year as an option.[9]
Espanyol
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.[10] Due to Diego López' suspension, he debuted on 13 June as the 70th goalkeeper in the club's history in a 2–0 home win over Deportivo Alavés.[11]
Pafos
On 17 August 2022, Oier moved abroad for the first time in his career and signed for Pafos FC in the Cypriot First Division.[12] He featured in only two games during his spell.[13]
Andorra
Oier returned to Spain and its second tier on 29 August 2023, after agreeing to a two-year contract at FC Andorra.[14]
^Navarro, Cristina (31 January 2020). "Oficial: Oier ficha por el Espanyol" [Official: Oier signs for Espanyol]. Marca (in Spanish). Retrieved 7 July 2020.