Dan Biggar kicked 14 points as Ospreys came from behind to beat Scarlets 19-9 in a closely-fought Guinness PRO12 derby at the Liberty Stadium.
The victory, which puts Ospreys up to second in the table, was secured with the aid of a second-half penalty try.
Scarlets were not helped that they were shown three yellow cards to scrum Rob Evans, flanker Will Boyde and replacement wing Steff Evans.
Biggar kicked four penalties and a conversion to take his tally to 2,000 points for the Ospreys, while Scarlets' scores came with three first-half penalties from Rhys Patchell.
Both teams came into the match on the back of some good from. Ospreys, led by lock Alun Wyn Jones, had won seven games in a row across all competitions, while Scarlets had beaten Toulon in their last game.
It was a cagey opening 40 minutes with the visitors enjoying a 9-6 interval lead through three Patchell penalties.
Defences appeared to be on top as the both sides gave away penalties during their opening incursions into opposition territory. And the absorbing stalemate was not broken until Biggar kicked a 15th-minute penalty after the Scarlets transgressed at a scrum.
There was a blow for the away side when two of their more influential players Scott Williams and Aaron Shingler had to leave the field for treatment after a clash of heads. Williams returned but Shingler was unable to continue following an assessment.
Scarlets suffered a potentially more worrying setback when Rob Evans was sin-binned for a high tackle on wing Dan Evans.
But Ospreys could not take advantage of the extra man with Alun Wyn Jones being held up over the line following good work from flanker Boyde.
Ospreys earned a penalty on the 22 which Biggar looked sure to kick but the ball sailed the wrong side of the right upright and the hosts were punished for the miss with Patchell kicking penalties after 34 and 39 minutes as the Scarlets back-row continued to boss the breakdown.
But Ospreys had the last say of a nip-and-tuck first half with Biggar kicking a final-minute penalty from close to halfway.
The key moment of the second half came after 54 minutes when Ospreys were awarded a penalty try with Boyde penalised for dragging down a maul for which he received a yellow card. Biggar converted and then kicked a penalty six minutes later.
Any chance the Scarlets had of getting back into the game and even securing a losing bonus point were dashed when Steff Evans was sin-binned for dragging down a maul after 70 minutes.
Two minutes later Biggar kicked his fourth penalty as Ospreys secured their 11th straight home win in all competitions this season.