On June 28, 2007, Green was selected with the fifth overall pick in the 2007 NBA draft by the Boston Celtics. He was later traded to the Seattle SuperSonics alongside Wally Szczerbiak and Delonte West in exchange for Ray Allen and Glen Davis. On April 6, 2008, Green scored a career-high 35 points against the Denver Nuggets. Green made the NBA All-Rookie First Team in 2008 after averaging 10.5 points, 4.7 rebounds and 1.5 assists in 80 games. The Sonics were sold and moved to Oklahoma City before the start of the 2008–09 season, becoming the Thunder. In January 2009, Green hit his first career game-winner, lifting the Thunder to a 122–121 win over the Golden State Warriors. In 2009–10, Green played and started in all 82 games. On December 1, 2010, he set a new career high with 37 points against the New Jersey Nets.