Biggar restored the Ospreys' 11-point advantage by knocking over a simple penalty three minutes later and could have added a drop-goal soon after but the fly-half chose to pass to Hook, who badly miscued his attempt.
First Biggar's swift pass found James Hook drifting just outside his marker and the centre raced through a gap to send Shane Williams racing over for a try on his 100th Ospreys appearance.