Villarreal originally signed as an undrafted amateur free agent on November 6, 1998 with the Arizona Diamondbacks. The following season he began a four year minor league stint, playing with various Diamondbacks affiliates. Villarreal was called up to make his MLB debut on March 31, 2003 with the Diamondbacks. He pitched in a total of 86 games that season, which was a record for games pitched by a rookie. His next two seasons with the Diamondbacks were plagued by injury. He missed much of the 2004 season after having nerve transposition surgery on his elbow, and again missed part of the 2005 season with a rotator cuff injury.

On December 7, 2005 the Diamondbacks traded Villarreal to the Atlanta Braves. Villarreal would average 55 games over the next two seasons with the Braves, including 11 complete games and six shutouts. Villarreal was traded to the Houston Astros on November 16, 2007. Two months later he signed a two-year contract worth a reported $2.85 Million.

1. Full name: Óscar Eduardo Villarreal
2. Born: November 22, 1981
3. Birthplace: Nuevo León, Mexico
4. Uniform Number: 56
5. Position: Relief Pitcher
6. Bats/Throws: Switch/Right