Óscar Pereiro Sío (pronounced [ˈoskaɾ pe'rejro ˈsi.o]; born 3 August 1977) is a Spanish former professional road bicycle racer.

Pereiro is a former member of Porta da Ravessa (2000 to 2001), Phonak Hearing Systems (2002 to 2005), Caisse d'Epargne (2006 to 2009), and the Astana cycling team (2010).

In January 2014, Pereiro confessed on a radio show he sold this stage to Hincapie, making the deal some kilometers before arriving to the finish line.

His breakaway Stage 13 second-place finish (just behind Germany's Jens Voigt) gained him almost 30 minutes on most of the General classification leaders and propelled him into an unexpected yellow jersey.

Explaining to Kimmage his satisfaction with his performance in the Stage 17 breakaway, he said, "I haven’t said this before because no one has asked, but if I had any reason to believe, and didn’t have specific knowledge that Pereiro was also doped, then I would have felt like I was cheating somebody - but I knew it, I had seen it first hand.

"[6] On 18 January 2007, French newspaper Le Monde reported that Pereiro had returned an "Adverse Analytical Finding" (AAF) during the 2006 Tour de France.

It is alleged that a concentration of salbutamol in excess of the threshold of the allowed therapeutic use was found in two urine samples, produced after stages 14 (Montélimar - Gap, in which Pereiro finished 26th) and 16 (Bourg-d'Oisans - La Toussuire, 3rd place).