The two sides had shared the spoils when they met in the pool stages in November with the Ospreys edging the verdict 10-7 in the opening game of the season between the sides.
However, it was the Scarlets who threatened to spoil the party as they led 10-9 at the interval courtesy of tries from flanker Laurence Renalds and scrum half Aled Davies.
Full back Tom Prydie, the Ospreys’ leading scorer this season, kicked three penalties to keep the holders within a point at the interval.
Scarlets captain and No.8 Steffan Phillips redressed the balance once again with a try shortly after half time, but after that, it was the Ospreys who took control.
Prydie’s fourth penalty brought the holders to within three points and when Gareth Harvey crossed for a try on the hour, Prydie stepped forward to convert for a 19-15 lead.
The Scarlets pressed once again in the final quarter, but replacement scrum half Ryan Jones made sure it was a black and white day at St Helens with a try that Prydie converting for a 26-15 victory.
