The Scarlets, who were without skipper Simon Easterby – a late withdrawal with a back injury – were kept on the back foot by the Ospreys who shut out the home side in a rain swept second half.
Gavin Henson was in inspired form for the visitors, producing some great positional kicks and collecting 18 points, including one of two second half tries. The Ospreys had built a 9-6 interval lead with Henson slotting three penalties, against two kicks from Arwel Thomas, but the lead could have been more as chances were spurned, especially when the Scarlets were reduced to 14 men by the carding of Salesi Finau for a stamp.
However the second half was one way traffic as, after more chances were wasted, the Ospreys finally crossed the Scarlets line when Henson gathered a loose ball in his own half, kicked ahead and won the footrace to the touch down.
As the rain intensified the Ospreys continued to pile on the pressure and in the final quarter left the champions reeling as Richie Pugh was on hand to gather Shane Williams pass and dive over for the try which sealed a superb away victory.