On 8 November 2009, Osmar made his debut with the Cantabrians' first-team, starting in a 2–0 La Liga home loss against Athletic Bilbao.

On 26 February 2013, Osmar made his debut in the AFC Champions League, in a match against Vegalta Sendai, acting as team captain and scoring a goal in the 1–1 away draw.

On 1 May, Buriram became the first team from the nation in a decade to reach the knockout rounds of the AFC Champions League,[5] and he topped his side's scorers list at three.

[6] On 31 December 2013, Osmar moved teams and countries again, joining FC Seoul on a three-year deal and becoming the first Spanish footballer to ever play in South Korea.

[7] Ahead of the 2015 season he was appointed vice-captain,[8] appearing in the all matches and minutes to set a new record for a foreign field player in the K League 1.