Three second-half penalties from Dan Biggar helped the Ospreys to a comfortable 21-14 win over the Scarlets at Parc y Scarlets on Boxing Day.
Man of the match Paul James scored in each half to condemn the hosts to their seventh defeat from nine outings.
The win takes the Ospreys top of the table for now, although that could change later in the day.
But the Scarlets - in front of 14,800 spectators, the first capacity crowd at their new ground - did sneak a losing bonus point at the death with flanker Johnathan Edwards going over in the final minute.
The Scarlets enjoyed territorial advantage in the first half and they deserved to go in at the interval with a 9-7 lead.
It was a typical Boxing Day derby with no quarter given - and a lack of Christmas friendship, with tempers often overflowing - but surprisingly the cards stayed in referee Nigel Owen's pocket.
While the Scarlets had the bulk of possession, they found it hard to break down the Ospreys defence as Alun-Wyn Jones, Jerry Collins and Ian Gough put in some big hits.
The Scarlets opened the scoring in the sixth minute with a penalty goal for Stephen Jones after Ricky Januarie was caught in possession.
The Ospreys had a chance to level matters with their own penalty attempt but Dan Biggar was wide of the target - his first miss in 19 attempts in the last three matches.
But it mattered little for the Ospreys, who scored from their first genuine attack. First Tommy Bowe and Biggar made good ground and, from a loose ball at the subsequent line-out, James went over unopposed for his first try.
The Scarlets responded well at the end of the first half. First Rhys Priestland put over a penalty from the halfway line and, in injury time, Stephen Jones gave his side a two-point lead at the break with his second penalty.
But the Ospreys produced a key moment three minutes into the second half when James, situated on the right wing, went over for the second and crucial try.
The Ospreys, boosted by the appearance of Lee Byrne coming on for his first rugby for two months, opened up an 18-9 lead with two Biggar penalty goals in the space of five minutes - the first which he missed but was allowed to retake 10 metres closer to the posts because Martin Roberts put him off.
The Scarlets twice threatened the Ospreys line with concerted attacks, but five minutes from time Biggar planted a third penalty.
Edwards' consolation try came from the final play of the game.
For the Scarlets:
Pens: Jones 2, Priestland
For the Ospreys:
Tries: James 2
Pens: Biggar 3
Scarlets: 15 Rhys Priestland, 14 Morgan Stoddart, 13 Sean Lamont, 12 Jonathan Davies, 11 Andy Fenby, 10 Stephen Jones, 9 Martin Roberts, 8 David Lyons (c) 7 Johnathan Edwards, 6 Simon Easterby, 5 Dominic Day, 4 Lou Reed, 3 Deacon Manu, 2 Ken Owens, 1 Phil John.
Replacements: 16 Jamie Corsi, 17 Emyr Phillips, 18 Vernon Cooper, 19 Rob McCusker, 20 Lee Williams, 21 Daniel Evans, 22 Gareth Maule.
Ospreys: 15 Gareth Owen, 14 Tommy Bowe, 13 Andrew Bishop, 12 James Hook, 11 Nikki Walker, 10 Dan Biggar, 9 Ricky Januarie, 8 Ryan Jones (c), 7 Marty Holah, 6 Jerry Collins, 5 Alun-Wyn Jones, 4 Ian Gough, 3 Craig Mitchell, 2 Huw Bennett, 1 Paul James.
Replacements: 16 Richard Hibbard, 17 Adam Jones, 18 Ian Evans, 19 Jonathan Thomas, 20 Jamie Nutbrown, 21 Sonny Parker, 22 Lee Byrne.
Referee: Nigel Owens