Wasteful play in final quarter cost Ospreys
Flyhalf Dan Biggar did his international chances no harm at all as he kicked 14 points to help give Ospreys the local bragging rights in a bad-tempered Pro12 derby which saw the Scarlets end proceedings with 12 men on the field.
Biggar is seemingly in the box seat to wear the No.10 jersey for next year's Six Nations after Rhys Priestland was hit by an Achilles injury.
And, with Priestland absent from the Scarlets line-up at the Liberty Stadium, Biggar further strengthened his cause with an impressive display in a 32-3 win.
The Ospreys had won their last five clashes with Welsh regions heading into the derby encounter and, thanks largely to their set-piece superiority, they kept that run going on Wednesday.
But for some wasteful play in the final quarter the Ospreys could have been celebrating a bonus-point win after Aled Thomas (yellow) and replacements Gareth Owen and Rob McCusker (both red) all fell foul of referee Nigel Owens late on.
In the end the home side had a penalty try plus tries from James King and Kahn Fotuali'i (who also slotted a drop-goal) to add to Biggar's haul with the boot.
The Ospreys were ahead as early as the second minute with a Biggar penalty from in front of the posts, after the Scarlets were penalised at scrum time.
It was 6-0 on 11 minutes when Kahn Fotuali'i added a drop-goal to his team's tally as Ospreys dominated the early exchanges.
The Ospreys had the advantage at the set piece and they made that pay on 24 minutes when referee Owens awarded them a penalty try.
Biggar again made no mistake with the conversion and Scarlets found themselves 13-0 down.
That margin was reduced two minutes later when a Thomas penalty got the Scarlets up and running. However there was more bad news for the West Walians just before the half-hour mark.
Second row forward George Earle was sent to the sin-bin to heap further pressure on their pack however it was 3-3 - a Thomas and a Biggar penalty - while the South African was off the field.
Earle re-entered the fray a minute before the interval only for the Ospreys - through a Biggar penalty taking his first-half tally to 11 points - to extend their lead to 19-3.
The home side started where they left off after the break and had a second try when King crossed in the corner.
Kahn Fotuali'i caught the Scarlets napping with a quick tap penalty and when the ball was recycled wide the second row, King, crossed.
Biggar missed the conversion but did nothing of the sort with a penalty on 50 minutes to make it 27-3.
The Ospreys got their third try when Fotuali'i touched down, just, in the corner, after holding off Scarlets flyhalf Thomas' tackle, Biggar missing the conversion.
The Scarlets No. 10's petulance then saw him given a yellow-card in the 71st minute as the Ospreys looked for that all-important fourth score.
The visitors ended the game with 12 when McCusker and Owen were both red-carded late on but the Ospreys weren't able to add a fourth try.
Tries: Penalty try, King, Fotuali'i
Pens: Biggar 4
Yellow cards: George Earle (Scarlets, 29), Tavis Knoyle, (Scarlets, 65), Aled Thomas (Scarlets, 72)
Red cards: Rob McCusker (Scarlets, 79), Gareth Owen (Scarlets, 80)
Scarlets: 15 Liam Williams, 14 George North, 13 Jonathan Davies (captain), 12 Scott Williams, 11 Kristian Phillips, 10 Aled Thomas, 9 Aled Davies, 8 Kieran Murphy, 7 Aaron Shingler, 6 Josh Turnbull, 5 George Earle, 4 Jake Ball, 3 Samson Lee, 2 Matthew Rees, 1 Phil John.
Replacements: 16 Emyr Phillips, 17 Shaun Hopkins, 18 Jacobie Adriaanse, 19 Sione Timani, 20 Rob McCusker, 21 Tavis Knoyle, 22 Gareth Owen, 23 Gareth Maule.
Ospreys: 15 Richard Fussell, 14 Ross Jones, 13 Andrew Bishop, 12 Ashley Beck, 11 Eli Walker, 10 Dan Biggar, 9 Kahn Fotuali'i (captain), 8 Joe Bearman, 7 Justin Tipuric, 6 Ryan Jones, 5 James King, 4 Ian Gough, 3 Adam Jones, 2 Richard Hibbard, 1 Ryan Bevington.
Replacements: 16 Scott Baldwin, 17 Duncan Jones, 18 Cai Griffiths, 19 Lloyd Peers, 20 Morgan Allen, 21 Rhys Webb, 22 Matthew Morgan, 23 Tom Isaacs.
Referee: Nigel Owens (Wales)
Assistant Referees: Gwyn Morris, Jon Mason (both Wales)
TMO: Derek Bevan (Wales)