Collison was selected by the Seattle SuperSonics with the 12th overall pick in the 2003 NBA draft, but missed the 2003–04 season with injuries to both shoulders. He made his NBA debut on November 3, 2004, recording two points, five rebounds and two assists in a 114–84 loss to the Los Angeles Clippers. He appeared in all 82 games in his rookie season, averaging 5.6 points and 4.6 rebounds in 17.0 minutes per game. He played in all 82 games for a second time during the 2006–07 season. On January 9, 2007, he had a career-best game with 29 points and 21 rebounds in a 113–102 loss to the Phoenix Suns. In 2008, the franchise relocated to Oklahoma City and rebranded as the Thunder. In 2012, he helped the Thunder reach the NBA Finals, where they lost in five games to the Miami Heat.