Scrum-half Mike Phillips celebrated his first start for nine months to score a try in the Ospreys' 20-6 win over the Scarlets.
The Welshman, who had surgery on his knee ligaments in March, crossed on the hour mark for what turned out to be the winning score after the home side had trailed 6-3 at half-time.
A second try for Wales captain Ryan Jones five minutes into injury time denied the Scarlets a losing bonus point.
After a frenetic start the Scarlets opened the scoring in the third minute with a Rhys Priestland penalty after the Ospreys had erred offside.
The Scarlets doubled their lead seven minutes later with a second Priestland penalty from in front of the posts after referee Nigel Owens had to warn home lock Jonathan Thomas after tempers were lost.
The Ospreys also spurned the best tryscoring chance when Gavin Henson broke through up the left and on 30 minutes James Hook was off-target with a penalty goal shot from 30 metres away. But five minutes later Hook found his range planting a penalty after Mark Jones and Regan King had collided trying to collect a Priestland cross-kick.
Seven minutes into the second half blood replacement Marty Holah crossed the line but the score was ruled out for a knock-on from Hook.
But on the hour mark the try stalemate was broken when Phillips skipped past two would-be tacklers to slide in by the posts for a converted try to give the Ospreys a 10-6 lead.
As the match entered its last 10 minutes Hook planted his second penalty to extend the Ospreys' advantage.
And at the death Ryan Jones went in under the posts for the second converted try.