The Ospreys are now just four points off top spot in the RaboDirect PRO12 standings after battling past the Scarlets 17-12 at Liberty Stadium.
Wales centre Ashley Beck and Fijian flier Aisea Natoga both touched down for Steve Tandy's men who withstood a late comeback to complete the double over their Welsh rivals.
The win lifted the Ospreys above Leinster into second place while a losing bonus=point saw the Scarlets rise to seventh place.
Despite the defeat, Scarlets started well and the visitors had the chance to take the lead as Ospreys were penalised for not releasing but Welsh international Rhys Priestland saw his effort held up by the swirling wind in south Wales.
After a tight opening quarter, Ospreys took the lead as Rhys Webb broke through the Scarlets defence before offloading to full-back Sam Davies who eventually found Beck who pounced over the try-line. Biggar kicked the extras to give the hosts a seven point advantage, with the Scarlets struggling to create any try-scoring opportunities.
Priestland got the visitors on the score-board with a penalty on the half hour mark but the Ospreys continued to control the possession.
Ospreys were awarded a penalty on the stroke of half-time and Biggar sent a long kick deep into the Scarlets 22 with second left in the first-half. The fly-half's gamble paid off as the Ospreys pack set up a driving maul before feeding the ball to the back, with Davies throwing a long pass to Natoga who touched down on the right wing. Biggar nailed the conversion in added time to give the Ospreys a 14-3 lead at the interval.
Scarlets had the chance to hit-back early on in the second half as Ben John was penalised for not rolling away but Priestland's long-range effort failed to find the posts.
His opposite number Biggar was having no such problems from the boot as he split the posts with a penalty minutes later to open up a 14 point lead.
Priestland eventually found his range and kicked his second penalty of the evening 13 minutes into the second period with neither side able to take a hold of the game.
Ospreys introduced Tom Habberfield and Lions hero Adam Jones as they looked to see the game out but Priestland kicked his third penalty of the night to cut the Ospreys' lead to eight points as the game ticked into the final quarter.
Simon Easterby's side enjoyed a spell of possession with Rob McCusker and George Earle both gaining valuable yards but they failed to find a way through a watertight Ospreys defence.
Priestland continued the Scarlets charge with an impressive line break deep into the Ospreys 22 before releasing Frazier Climo but the New Zealand winger was shoved into touch as another chance went begging.
With 10 minutes left on the clock, try-scorer Natoga was yellow carded for a deliberate knock on but the Ospreys continued to throw their bodies on the line in a superb defensive effort.
Priestland ensured a nervy finish to proceedings in Swansea with his fourth penalty five minutes from time but the Ospreys held firm to secure a vital victory.
Tries: Beck, Natoga
Con: Biggar, Davies
Pen: Priestland 4
Ospreys: 15 Sam Davies, 14 Aisea Natoga, 13 Ben John, 12 Ashley Beck, 11 Jeff Hassler, 10 Dan Biggar, 9 Rhys Webb, 8 Joe Bearman, 7 Sam Lewis, 6 Justin Tipuric, 5 Ian Evans, 4 Alun Wyn Jones, 3 Aaron Jarvis, 2 Richard Hibbard, 1 Duncan Jones
Replacements: 16 Scott Baldwin, 17 Ryan Bevington, 18 Adam Jones, 19 James King, 20 Morgan Allen, 21 Tyler Ardron, 22 Tom Habberfield, 23 Richard Fussell
Scarlets: 15 Gareth Owen, 14 Kristian Phillips, 13 Nick Reynolds, 12 Scott Williams, 11 Frazier Climo, 10 Rhys Priestland, 9 Rhodri Williams, 8 Rob McCusker (c), 7 John Barclay, 6 Josh Turnbull, 5 George Earle, 4 Jake Ball, 3 Samson Lee, 2 Emyr Phillips, 1 Phil John
Replacements: 16 Kirby Myhill, 17 Rob Evans, 18 Jacobie Adriaanse, 19 Johan Snyman, 20 Aaron Shingler, 21 Gareth Davies, 22 Aled Thomas, 23 Jordan Williams.
Referee: Nigel Owens (Wales)
Assistant referees: Ian Davies, Jon Mason (both Wales)
Television match official: Derek Bevan (Wales)