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.
The Ospreys, who were boosted by the return of Wales prop Adam Jones for his first start in two months, were dominant throughout as they sought to build on their 17-6 Heineken Cup win over Toulouse last time out.
The home team struck the first blow when Biggar kicked a third-minute penalty from a scrum offence.
Good pressure from the Ospreys, who hammered at the Scarlets' 22, ended with a snap drop-goal from scrum-half Fotuali'i to double their lead.
Centre Ashley Beck's kick to the corner further racked up the pressure on the Scarlets, who lost the resulting line-out and were forced to defend a five-metre scrum.
But the Scarlets scrum disintegrated as referee Nigel Owen gave the home side a penalty try converted by Biggar.
There was some respite for the Scarlets when fly-half Thomas opened the visitors' account with a 27th-minute penalty.
However, the Scarlets, showing four changes from their win over Cardiff Blues last week, found themselves on the back foot two minutes later when lock George Earle was yellow-carded for an illegal tackle on Fotuali'i, who had taken a quick tapped penalty.
The 14-men Scarlets escaped a period of intense pressure. First Scott Williams prevented Ryan Jones from scoring before Biggar lost the ball in a tackle.
Jones went over the line again but referee Owen had already blown for a penalty which Biggar kicked to give the Ospreys a 16-3 lead on 34 minutes, with his opposite number Thomas missing his own shot at goal two minutes later.
A forgettable first half for the Scarlets was made worse for them when Biggar put over his third penalty in injury time to give the Ospreys a 16-point interval lead.
The Ospreys carried on in the second half where they left off, scoring an unconverted try for King on 48 minutes after Richard Fussell had broken down the left flank.
And a 21-point lead was increased to 24 with a fourth Biggar penalty.
Matters did not get any better for the Scarlets as replacement scrum-half Knoyle was sin-binned for taking Eli Walker without the ball as Fotuali'i squeezed in at the left corner.
The Scarlets had a third man, fly-half Thomas, sent to the bin before a dramatic final three minutes saw McCusker and Owen both shown red for foul play.
- Sports & Recreation
- Dan Biggar