The Ospreys made it successive wins over the RaboDirect PRO12 festive season after they edged out the Scarlets 10-6 in Llanelli on Thursday.
Steve Tandy's side came from behind to win a bruising encounter at a sold-out Parc y Scarlets.
Replacement Ian Evans scored the only try of the match in the final minutes of an intense game after the Ospreys had trailed from the fourth minute.
A pair of penalties from Rhys Priestland looked to have sealed the game for the hosts but the visitors eventually broke down their opponents in the final quarter to seal a crucial victory.
Scarlets took the lead in a frenetic opening period in west Wales as Priestland split the posts with a penalty after Ryan Jones was penalised for not releasing after six minutes.
The hosts came within a whisker of doubling their advantage after the Ospreys were penalised for standing in the scrum but New Zealand winger Frazier Climo saw his long-range penalty bounce back off the post.
Ospreys dominated possession but Scarlets defended resolutely as Dan Biggar let fly with a 30 metre drop-goal but his effort sailed wide of the posts.
Scrum-half Rhys Webb made a clean break into the Scarlets' 22 as tackles continued to fly in all over the pitch but neither side threatened to score in a brutal first-half.
Biggar came close to levelling five minutes before the break, after a disagreement over the placing of the ball with referee Leighton Hodges, but he saw his kick fall inches short, his first miss in 36 attempts.
After a well-disciplined opening 40 minutes, George Earle conceded an early second-half penalty but Biggar fluffed his lines from close-range as Simon Easterby's side held on to their slender lead.
Ospreys lock Alyn-Wyn Jones was penalised minutes later for taking the man out in the air at the line-out and Priestland slotted his second penalty of the match to double his side's lead.
Ospreys continued to dominate the ball but it was the home side who looked the more threatening with the impressive Liam Williams being central to a superb counter-attacking game plan.
Tandy's men eventually broke the Scarlets defence with a sweeping line-out move and Hanno Dirksen came agonisingly close to scoring the game's first try, but his effort was eventually ruled out by the TMO.
The visitors were rampant but Scarlets were throwing bodies on the line, as Samson Lee and Phil John were both replaced after Ken Owens had been withdrawn with a serious leg injury.
Biggar eventually got the Ospreys on the score-board with only 13 minutes left on the clock to reduce the deficit to three points.
And the visitors finally made the breakthrough with only seven minutes remaining as Biggar launched an inch-perfect penalty into touch to allow Evans to score from the driving maul.
Biggar nailed the conversion and the Ospreys withstood a string of attacks, despite seeing Alun-Wyn Jones being sent to the sin-bin, to secure a crucial win.
Pen: Priestland 2
Scarlets: 15 Liam Williams, 14 Kristian Phillips, 13 Nick Reynolds, 12 Scott Williams, 11 Frazier Climo, 10 Rhys Priestland, 9 Gareth Davies, 8 Rob McCusker (c) 7 John Barclay, 6 Aaron Shingler, 5 George Earle, 4 Jake Ball, 3 Samson Lee, 2 Ken Owens, 1 Phil John.
Replacements: 16 Emyr Phillips, 17 Rob Evans, 18 Rhodri Jones, 19 Richard Kelly, 20 John Snyman, 21 Rhodri Williams, 22 Aled Thomas, 23 Gareth Maule.
Ospreys: 15 Sam Davies, 14 Hanno Dirksen, 13 Ashley Beck, 12 Ben John, 11 Jeff Hassler, 10 Dan Biggar, 9 Rhys Webb, 8 Ryan Jones, 7 Sam Lewis, 6 Joe Bearman, 5 James King, 4 Alun Wyn Jones (c), 3 Adam Jones, 2 Richard Hibbard, 1 Duncan Jones.
Replacements: 16 Scott Baldwin, 17 Marc Thomas, 18 Aaron Jarvis, 19 Ian Evans, 20 Morgan Allen, 21 Tyler Ardron, 22 Tito Tebaldi, 23 Tom Isaacs.
Referee: Leighton Hodges (Wales)
Assistant Referees: Sean Bricknell (Wales), Wayne Davies (Wales)
Citing Commissioner: Russell Howell (Wales)
TMO:Tim Hayes (Wales)