Wales' precocious young fly-half James Hook once again showed his potential with a match-winning 19-point haul for the Ospreys against the Scarlets on Tuesday, with the Ospreys triumphing 6-19 in the Magners League.
Hook scored a try, three penalties, a drop goal and a conversion to keep Ospreys in the hunt in the Magners League.
Llanelli's only reply came from the boot of Stephen Jones who kicked two penalties.
Hook's efforts moved Ospreys up to third in the league table with a potential title decider against Leinster to come at the Liberty Stadium on Friday night.
The victory, based on a magnificent defensive effort, completed the derby double for Lyn Jones' side and also eliminated the Scarlets from the title race for this season - another blow after their Heineken Cup exit on Saturday.
Both sides had gone into the game having been tamed by Leicester Tigers in their previous matches - the Ospreys in the EDF Energy Cup final at Twickenham and the Scarlets in the Heineken Cup semi-final at the Walkers Stadium.
There never is much love lost between these sides and so it proved in a fractious opening which saw Scarlets skipper Simon Easterby and Ospreys lock Brent Cockbain pulled apart in the first minute.
A monster 55-metre penalty from Hook minutes later put the visitors on the scoreboard, before Hook struck again after more home indiscipline.
Opposite number Stephen Jones responded with a penalty of his own after 11 minutes but the Scarlets then turned down another kickable chance in favour of close-range line-out.
It was proving a typical derby occasion. Plenty of passion, plenty of penalties and plenty of incidents.
But unlike the 50-24 points feast at the Liberty Stadium over Christmas, not many try-scoring opportunities were created - the damp conditions played their part as well.
The Ospreys came close when former All Black scrum-half Justin Marshall broke clear, then it needed a big hit from Sonny Parker on Scarlets young centre Gavin Evans to prevent the home side crossing.
A second Jones effort brought the scores level after 31 minutes, only for Hook to strike over his third a minute before the interval.
The Ospreys had abrasive Kiwi back-rower Filo Tiatia sin-binned in first-half stoppage time - the victim of a string of infringements by his team-mates, but Jones pushed the penalty attempt wide with the last kick of the half.
It remained tight until the hour mark only for the game to then turn on a moment of magic by Wales wing Shane Williams.
Picking up turnover ball inside his own 22 the twinkle-toed flyer burst clear of four defenders before linking up with Hook at the other end of the field.
After some slick handling by the Ospreys midfield, Hook looped around to dive in at the corner and the Ospreys were in command.
Hook added the touchline conversion and a smart drop-goal shortly after, leaving the Scarlets with too much to do in the closing stages.
To their credit, they threw everything at the Ospreys line in the final minutes, but the visitors, even when replacement Mike Powell was sin-binned, held out superbly to claim what could be a crucial victory in the title race.
For the Scarlets:
Pens: Stephen Jones 2
For the Ospreys:
Pens: Hook 3
Drop goal: Hook
Yellow cards: Tiatia (40, Ospreys), Powell (74, Ospreys)
Scarlets: 15 Barry Davies, 14 Mark Jones, 13 Regan King, 12 Gavin Evans, 11 Matthew Watkins, 10 Stephen Jones, 9 Dwayne Peel, 8 Dafydd Jones, 7 Gavin Thomas, 6 Simon Easterby, 5 Scott MacLeod, 4 Adam Jones, 3 Craig Dunlea, 2 Matthew Rees, 1 Iestyn Thomas,
Replacements: 16 Deacon Manu, 17 Ken Owens, 18 Alix Popham, 19 Inoke Afeaki, 20 Liam Davies, 21 Ceiron Thomas, 22 Darren Daniel.
Ospreys: 15 Lee Byrne, 14 Nikki Walker, 13 Sonny Parker, 12 Andrew Bishop, 11 Shane Williams, 10 James Hook, 9 Justin Marshall, 8 Filo Tiatia, 7 Richard Pugh, 6 Ryan Jones, 5 Jonathan Thomas, 4 Brent Cockbain, 3 Andrew Millward, 2 Barry Williams, 1 Duncan Jones (c).
Replacements: 16 Huw Bennett, 17 Cai Griffiths, 18 Mike Powell, 19 Lee Beach, 20 Jason Spice, 21 Shaun Connor, 22 Stefan Terblanche.