Özbilen, then competing as William Biwott Tanui, won the 1500 metres at the 2009 World Athletics Final in Thessaloniki, Greece. He holds the current world junior record in the mile run with his clocking of 3:49.29 minutes, achieved in Oslo on 3 July 2009. He was also part of the Kenyan team (with Gideon Gathimba, Geoffrey Kipkoech Rono and Augustine Kiprono Choge) that set a new world record of 14:36.23 in the rarely contested 4 x 1500 metres relay at the 2009 Memorial Van Damme Golden League meeting in Brussels, Belgium.