William Oloonkishu Yiampoy (born 17 May 1974 in Emarti) is a Kenyan runner who specializes in the 800 metres. His personal best is 1:42.91 minutes, achieved in September 2002 in Rieti.
He graduated from Sosio High School in Kilgoris in 1989. He was recruited by Kenya Police in 1991. He did not start running until 1996. In 1999 he ran his first European races. He was selected to compete at the 2000 Olympics replacing Patrick Konchellah, the winner of Kenyan Olympic trials.