On 24 August 2014 he played his first match as a professional, replacing Bernardo in the 85th minute of a 0–1 away loss against Girona FC in the Segunda División.
[3] Óscar only featured in one more match for the first team during the season, a 1–2 home loss against CD Leganés on 21 February 2015;[4] his side subsequently suffered relegation.
He was definitely promoted to the main squad the following July, and scored a brace in a 2–0 Segunda División B home win against Cultural y Deportiva Leonesa on 20 March 2016.
[5] On 17 July 2018, free agent Óscar signed a two-year deal with AD Alcorcón, but was immediately loaned to CF Fuenlabrada for one year.
[8] After seven months at CD Don Benito from January to July 2020, Fernández moved to Pontevedra CF on a deal until June 2021.