|Full name||Óscar Guido Trejo|
|Date of birth||26 April 1988|
|Place of birth||Santiago del Estero, Argentina|
|Height||1.77 m (5 ft 10 in)|
|2009–2010||→ Elche (loan)||34||(4)|
|2010–2011||→ Rayo Vallecano (loan)||40||(9)|
|* Senior club appearances and goals counted for the domestic league only and correct as of 20 May 2017.
A product of Boca Juniors' youth system, Trejo only played once for its first team but, in the game against Club Almagro, he scored in a 2–3 away loss after a mere minutes on the pitch. The match was suspended after 63 minutes.
In late December 2006, Trejo was bought by RCD Mallorca in Spain, still having not signed his first professional contract. Boca sustained otherwise and the case was taken to FIFA, which ultimately decided for the Balearic Islands club. He made his first appearance in La Liga on 8 April 2007, against Getafe CF: having come on as a 90th-minute substitute, he managed to find the net for a final 2–0 home victory.
Sparingly used in the following seasons, Trejo managed to score in the 2007–08 edition of the Copa del Rey in a 2–1 home win over Real Madrid (3–1 aggregate). After an unassuming 2008–09, he was loaned for the following campaign to second division side Elche CF.
On 15 August 2010 a similar move ensued, with Trejo joining Rayo Vallecano in the same level. He returned to the top flight in the 2011–12 season, scoring four goals in 33 contests for Sporting de Gijón but suffering team relegation.
In the 2012 summer, Premier League's West Ham United made an attempt to buy Trejo, but the deal fell apart. One year later he eventually left Spain, going on to spend several years in the French Ligue 1 with Toulouse FC.
- Boca Juniors
- Un grupito hizo lío y partido suspendido (Hooligans made a mess and match suspended); Clarín, 4 July 2005 (in Spanish)
- La FIFA da la razón al Mallorca en el 'caso Óscar Trejo', que será presentado mañana oficialmente (FIFA decides for Mallorca in the 'Óscar Trejo Affair', he will be presented officially tomorrow); Terra, 5 March 2007 (in Spanish)
- "El debutante Trejo sentencia al Getafe con un golazo" [Debutant Trejo finishes Getafe with wonder goal] (in Spanish). El País. 8 April 2007. Retrieved 28 December 2015.
- "El argentino Óscar Trejo dice que "ojalá" pueda jugar más minutos" [Argentine Óscar Trejo says that "hopeully" he will play more minutes] (in Spanish). Soitu. 16 April 2008. Retrieved 24 November 2013.
- Spain Cup brings misery to Madrid; UEFA.com, 10 January 2008
- "Óscar Trejo jugará en el Elche cedido por el Mallorca" [Óscar Trejo will play in Elche loaned by Mallorca] (in Spanish). Marca. 22 August 2009. Retrieved 24 November 2013.
- "Óscar Trejo llega cedido al Rayo" [Óscar Trejo arrives to Rayo on loan] (in Spanish). Marca. 15 August 2010. Retrieved 24 November 2013.
- "Rayo Vallecano land Mallorca attacker Oscar Trejo". Tribal Football. 16 August 2010. Retrieved 19 August 2010.
- "Óscar Trejo ya piensa en sportinguista" [Óscar Trejo already thinks as sportinguista] (in Spanish). Sporting de Gijón. 17 June 2011. Retrieved 24 November 2013.
- Simon Jones (18 July 2013). "Hammers considering £1.8m offer for Sporting Gijon attacking midfielder Trejo". Daily Mail. Retrieved 12 August 2013.