|Scarlets (14) 22|
|Tries: L Williams, Barclay Pens: Priestland 4|
|Ospreys (3) 10|
|Tries: D Evans Con: Biggar Pen: Biggar|
Scarlets claimed a first win over Ospreys since 2011 with a dominant display at Parc y Scarlets.
Liam Williams' well-worked try and three Rhys Priestland penalties gave the hosts a 14-3 half-time lead.
John Barclay's fine team try after the restart and another Priestland penalty extended their advantage to 22-3.
Dan Evans wrestled his way over to give Ospreys hope, but Scarlets held firm to end a run of five successive defeats against their West Walian rivals.
Victory was richly deserved for Scarlets, who overpowered Pro12 leaders Ospreys up front and outmanoeuvred them behind the scrum with a sharp attacking display.
Ospreys were dealt a blow shortly before kick-off when Wales scrum-half Rhys Webb was ruled out with illness and replaced by Martin Roberts.
With that late alteration and Aisea Natoga coming in for injured wing Eli Walker, Ospreys made two changes from the team that beat Scarlets 17-15 the previous weekend.
Scarlets made five changes, captain Scott Williams the most notable recall following a shoulder injury.
The Wales centre was immediately at the heart of the action, making a searing break inside the first minute which led to the game's first points courtesy of a Priestland penalty.
Williams' run sparked the vociferous Parc y Scarlets crowd into life, making for a charged atmosphere during a bruising and high-octane opening.
But the Scarlets skipper's next significant involvement prompted howls of fury rather than roars of approval from the home fans, who were unhappy with referee John Lacey's decision to sin-bin Williams for a high tackle on Ospreys hooker Scott Baldwin.
During Williams' spell on the sidelines, the visitors drew level thanks to a Biggar penalty but, moments after the Scarlets captain's return, Priestland kicked the hosts back in front.
Scarlets were worth their lead having controlled possession and territory for much of the first half, and their domination culminated in the game's opening try after half an hour.
Priestland was the architect, weaving his way through the Ospreys' midfield before passing to Liam Williams, who finished gleefully from close range for an unconverted score.
The home side were close to a second try four minutes later, but scrum-half Aled Davies knocked on as he attempted to collect a grubber-kick on the Ospreys' try line.
Scarlets led 14-3 at half-time but extended their lead just 40 seconds after the restart.
Scott Williams made another scintillating break from his own half into the Ospreys' 22, where he offloaded to his namesake Liam.
Although the full-back was tackled by Biggar, the ball was recycled quickly for Barclay to dive over for the hosts' second unconverted try.
That prompted a rare period of pressure from Ospreys and Justin Tipuric was just inches away from touching down from Hanno Dirksen's kick.
Scarlets were still in the ascendancy, however, and Priestland kicked his fourth penalty to stretch their advantage to 22-3.
A powerful scrum was the basis of the Scarlets' domination and, unlike their error-strewn display the previous week, the accuracy and guile of their backs gave their attacking play a cutting edge which had been missing at the Liberty Stadium.
Ospreys were way below their best, though they did give themselves hope of a comeback when full-back Evans wriggled his way over for a try despite the close attention of two Scarlets tacklers.
It proved to be little more than a consolation score for the visitors, however, as Scarlets saw out the closing stages in Ospreys territory to complete a comfortable victory.
Despite the defeat Ospreys stay top of the Pro12, after nearest rivals Glasgow and Munster both lost over the weekend, while Scarlets stay in seventh.
Scarlets: Liam Williams; Hadleigh Parkes, Regan King, Scott Williams (capt), Michael Tagicakibau; Rhys Priestland, Aled Davies; Rob Evans, Kirby Myhill, Samson Lee, Jake Ball, Joe Snyman, Aaron Shingler, John Barclay, Rob McCusker.
Replacements: Ryan Elias, Wyn Jones, Rhodri Jones, George Earle, Will Boyde, Gareth Davies, Steven Shingler, Jordan Williams.
Ospreys: Dan Evans; Hanno Dirksen, Ashley Beck, Josh Matavesi, Aisea Natoga; Dan Biggar, Martin Roberts; Marc Thomas, Scott Baldwin, Dmitri Arhip, James King, Alun Wyn Jones (capt), Dan Lydiate, Justin Tipuric, Tyler Ardron.
Replacements: Sam Parry, Duncan Jones, Aaron Jarvis, Rynier Bernardo, Sam Lewis, Sam Davies, Tom Grabham.
Referee: John Lacey (IRFU)
Assistant Referees: Ian Davies, Marty Lewis (both WRU)
Citing Commissioner: Aurwel Morgan (WRU)
TMO: Tim Hayes (WRU)