The Ospreys moved to the top of the Magners League, if only temporarily ahead of this evening's matches in Ireland, and looked ominously strong as they gradually quietened the largest crowd so far attracted to Parc y Scarlets.
The Scarlets remain one off the bottom of the table with two victories in nine matches. Only Connacht lie beneath them and the manner of today's defeat, which saw a liberal supply of primary possession turned into scraps at the breakdown, suggests hard times ahead.
But the Ospreys, who have group-deciding Heineken Cup matches against Clermont Auvergne and Leicester next month, were on a different level. The way they defended throughout despite having considerably less possession showed how they have become harder mentally and it was not until the ninth minute of stoppage time with the points long secured that their line was breached.
It was a typical Boxing Day derby: attritional, hard fought and lacking any seasonal goodwill. The Ospreys' South African scrum-half, Ricky Januarie, was fortunate an attempted head-butt on the Scarlets' wing, Matthew Stoddart, went undetected 30 minutes in.
Januarie had been having one of those games. He had twice wasted turnover possession by kicking the ball away aimlessly and two attempts at catching high kicks were so abject that the ball came closer to hitting his head than his hands.
His short-term loan may end prematurely if he is cited, although Stoddart's advantage in height saw the butt impact on his right shoulder. There were numerous skirmishes as Boxing Day lived up to its name and, worryingly for the Wales coach, Warren Gatland, ahead of the Six Nations Alun Wyn Jones and James Hook left the field with shoulder injuries.
The Wales full-back Lee Byrne made his return after two months out with a foot injury and his fellow Lion Adam Jones had his second outing as a substitute after shoulder surgery and a notable difference between the two sides was strength in reserve.
All bar one of the Ospreys' replacements were internationals while the Scarlets largely had tyros in occupation. And so, while the home side competed strongly in the opening half, they never once threartened to break the hold of a constricting defence.
The Scarlets led 9-7 at the break through two Stephen Jones penalties and one from halfway by Rhys Priestland, but the three meaningful attacks of the half came from the Ospreys, the last of which saw the prop, Paul James, dive through a pile of bodies after a lineout.
James had scored two tries in 136 previous appearances for the Ospreys and he doubled his tally three minutes after the restart, lurking on the right wing and having the pace and space to cross despite taking Dan Biggar's long pass on the bounce.
The Ospreys were going for the kill and controversially extended their lead to 15-9 on the hour. Biggar's 50-yard penalty drifted wide but the referee, Nigel Owens, told the outside-half to take the kick again 10 yards closer after penalising the home scrum-half, Martin Roberts, for passing comment as the original kick was being taken.
Indiscipline cost the Scarlets again three minutes later. They were awarded a penalty 25 yards out only for it to be reversed when Vernon Cooper was flagged for dangerous footwork. Biggar, who had become the game's authoritative figure through his kicking out of hand, landed his second and third penalties in between a siege of the visitors' line to take his side to the top of the table. Jonathan Edwards gave the Scarlets a bonus point with a try from quickly taken penalty with the final move of the game.
Scarlets: Priestland; Stoddart (Williams, 70), Lamont, Davies, Fenby; S Jones, Roberts; John, Owens, Manu, Reed, Day (Cooper, 51), Easterby (McCusker, 57), Edwards, Lyons (capt).
Try: Edwards. Pens: Jones 2, Priestland.
Ospreys: Owen (Byrne, 46); Bowe, Bishop, Hook (Parker, 40), Walker; Biggar, Januarie; James, Bennett (Hibbard, 52), Mitchell (A Jones, 39), Gough, AW Jones (J Thomas 43), Collins, Holan, R Jones (capt).
Tries: James 2. Con: Biggar. Pens: Biggar 3.
Referee: N Owens (Pontyberem)
Attendance: 14,739
