On June 9, Floyd sent a brief letter of resignation to Athletic Director Mike Garrett, officially ending a four-year coaching tenure at the Pac-10 Conference school.
O'Neill, 52, has 13 years of head coaching experience at the collegiate and NBA levels and was interim head coach at Arizona in 2008 when Lute Olson took a leave of absence for personal reasons.
"We're thrilled to have Kevin O'Neill as our men's basketball coach," Garrett in a prepared statement. "Kevin is the consummate coach. He knows his Xs and Os, he's an excellent recruiter and he is very in tune with the academic side of a player's collegiate experience. He is prepared for this opportunity."
At the college level, O'Neill has a combined 171-180 record.
He was working for the NBA's Memphis Grizzlies and also coached the Toronto Raptors.