De Marcos playing for Athletic in 2012
|Full name||Óscar de Marcos Arana|
|Date of birth||14 April 1989|
|Place of birth||Laguardia, Spain|
|Height||1.80 m (5 ft 11 in)|
|Playing position||Midfielder / Full back|
|* Senior club appearances and goals counted for the domestic league only and correct as of 14 March 2015.
† Appearances (Goals).
Born in Laguardia, Álava, de Marcos made his professional debuts for hometown's Deportivo Alavés, first appearing in a second division home fixture against CD Tenerife on 21 December 2008. He eventually played in 20 games with the first team, as it dropped down a level at the season's end; he started his career as a forward.
In July 2009 de Marcos went straight into La Liga, penning a four-year deal with Athletic Bilbao for about €350,000. He made his club debut on 6 August in a 2–1 win at BSC Young Boys for the UEFA Europa League (2–2 aggregate, victory on the away goals rule), as a starter; ten days later he opened the score against FC Barcelona for the season's domestic Supercup as the Basque eventually lost 1–2 at home and 1–5 in total.
Having been signed initially for the reserve squad, de Marcos had more impact than expected – as 17-year-old Iker Muniain – appearing in 29 official contests during the 2009–10 campaign and scoring three goals. He played his only match with the B-team on 14 April 2010 against Zamora CF, in a season where they narrowly avoided relegation from division three.
De Marcos appeared less in his second season, but was also used as a full back by manager Joaquín Caparrós, finishing the year with 15 games in all competitions. The following campaign, under Marcelo Bielsa, he played in that position and also in centre midfield.
On 17 December 2011, de Marcos played roughly one hour of the league fixture against Real Zaragoza (2–1 home win) with a tear in his scrotum, which later required 25 stitches. On 4 January of the following year he extended his contract with the Lions until June 2016, with a release clause of €32 million.