Gavin Henson inspired the Ospreys to a famous win at Stradey by scoring 18 of his side's points against the Scarlets.
Scarlets (6) 6: Pens: A Thomas 2.
Ospreys (9) 23: Tries: Henson, Pugh. Cons: Henson 2. Pens: Henson 3.
Henson struck three penalties from full-back before moving to fly-half after an injury to Matthew Jones.
In the second half Henson went the length of the field to score under the posts, converting both his and Richie Pugh's later try.
The Scarlets were under pressure all night and had just two Arwel Thomas penalties to show for their efforts.
The first quarter was a personal contest between the two fly-halves, with Matthew Jones laying down the gauntlet and Arwel Thomas picking up the challenge for the Scarlets.
The young Osprey kept the visitors driving forward with a series of raking kicks in behind the Scarlets defence.
Thomas' tactical kicking was no less precise, while a beautiful dink over the Ospreys' rush defence almost set centre Matthew Watkins free.
The place-kicking was a different battle, with Henson striking two early penalties either side of a brace from Thomas to leave the teams tied 6-6 after 21 minutes.
A cagey match burst into life following the sin-binning of Scarlets wing Salesi Finau for stamping on Ospreys prop Duncan Jones.
The Ospreys at last strung several phases together to threaten the Scarlets' try line, but lost fly-half Jones to a concussion in the process.
The youngster was put in a neck brace and stretchered off as a precaution, with Henson moving to No 10 and Adrian Durston coming on at full-back.
Henson added his third penalty, but the visitors should have had more than a three-point lead at half-time after wing Shane Williams dropped a pass with the line at his mercy.
The break saw the arrival of heavy rain, making handling difficult and the playing surface treacherous underfoot.
That could have been the reason why Henson missed a simple chance to stretch the Ospreys' lead after 52 minutes, hooking his kick left of the uprights.
But the Wales man redeemed himself in style within five minutes, as he started and finished a length-of-the-field counter to score under the posts.
The Scarlets had at last worked their way into the visitors' 22 but spilled the ball, allowing Henson to gather and launch a huge kick-and-chase upfield.
Finau, Aisea Havili and Barry Davies all raced back, but none could stop Henson from touching down and slotting an easy conversion to open up a 10-point gap.
It soon got worse for the Scarlets, when Jason Spice's intelligent kick from the base of a ruck was retrieved by Shane Williams and shipped on to flanker Pugh for a converted try.
The Scarlets showed their character to put huge pressure on the Ospreys line, with only a superb covering effort by prop Duncan Jones preventing Thomas from scoring.
Scrum-half Mike Phillips made a couple of excellent breaks, but each time either the Ospreys got back in enough numbers or poor handling snuffed out the danger.
Scarlets: B Davies; A Havili, M Watkins, M Taylor, S Finau; A Thomas, M Phillips; I Thomas, M Rees, J Thiel, V Cooper (capt), C Wyatt, D Jones, G Thomas, S Quinnell.
Replacements: J Davies, C Hawkins, A Jones, R Walters, T Selley, A Thomas, A Powell.
Ospreys: G Henson; S Terblanche, D Bishop, S Parker, S Williams; M Jones, J Spice; D Jones, B Williams (capt), A Millward, A Newman, B Cockbain, J Thomas, R Pugh, R Jones.
Replacements: A Williams, D Tiueti, A Durston, A Jones, Hibbard, L Bateman, A Lloyd.
Referee: Allan Lewis (Ireland).