Russell Westbrook wore number 0 throughout his UCLA career. He played for the Bruins from 2006 to 2008, using the unusual number after discovering that his preferred high-school number, 4, was already taken by teammate Arron Afflalo.
As a freshman, Westbrook was primarily a defensive specialist and energetic reserve behind Darren Collison. He became a starter as a sophomore and helped UCLA reach the Final Four in the 2007–08 season. His speed, defense, attacking style, and improving playmaking helped turn him into one of college basketball’s most explosive guards.
The number 0 later became closely associated with Westbrook in the NBA. The Seattle SuperSonics selected him fourth overall in the 2008 draft, and he kept number 0 with the Oklahoma City Thunder, Houston Rockets, Washington Wizards, and Los Angeles Lakers. He has also worn number 4 at times, including with the Washington Wizards and Los Angeles Clippers.
The question refers specifically to his UCLA career, so 0 is the canonical answer. It should not be confused with his high-school number 4 or with the number changes he made during his professional career.