After the Ospreys won at Parc y Scarlets in September they were again the dominant side with a penalty try and two further tries for lock James King and skipper Kahn Fotuali'i.
Fly-half Dan Biggar added 14 points with the boot and Fotuali'i also kicked a drop-goal as Ospreys stormed to victory in front of a crowd of 19,443 in Swansea.
The Scarlets were not helped that they had replacements Rob McCusker and Gareth Owen sent off in the final minutes and also had another three players sin-binned - George Earle, Tavis Knoyle and Aled Thomas.
