Man of the match Sam Davies kicked 14 points and Luke Morgan scored the only try of the game as the Ospreys came from behind to claim the derby spoils at the Liberty Stadium.
It was a real game of two halves as the Scarlets dominated the first half and led by three at the break, before the Ospreys took control in the second 40.
The away team drew first blood, Rhys Patchell splitting the posts from 35m out after six minutes, the Ospreys penalised at the first scrum of the afternoon.
It was level inside two minutes, Sam Davies from halfway after James Davies was pinged for offside.
The exchange of penalties continued when an offside call against Adam Beard and Olly Cracknell, in front of the kicker, allowed Patchell to nudge over his second penalty, Davies then replying again after namesake James was offside once more.
Patchell should have given the Scarlets the lead for a third time in the 18th minute, Scott Baldwin not rolling away, but his kick never looked on target, drifting harmlessly across the posts and wide.
The visitors were enjoying plenty of possession and territory as the game moved into the second quarter, without unduly troubling a well organised Ospreys defence.
However another scrum penalty allowed Patchell a straightforward shot at goal and he took his team three ahead with 27 minutes gone.
Opportunities had been at a premium for both sides but a better bounce could have made it interesting in the 38th minute as a Scott Williams grubber bounced invitingly but Luke Morgan couldn’t quite gather 20m out with empty space in front of him, the ball going off him into touch,
HALF-TIME: OSPREYS 6-9 SCARLETS
Sam Davies had the first sight of goal after the restart, a kick from 40m out wide on the right after some great work by George North chasing his Davies’ cross kick to win the penalty, but his shot was pulled short of the near post.
The third quarter was the complete reversal of what had gone before, the Ospreys dominating but like the Scarlets in the first half, struggling to create any real openings, their best spell coming after a great chase from Scott Williams’ kick through, North and Tipuric putting Steffan Evans under pressure on his own line to win the scrum.
The Scarlets survived a prolonged spell of pressure inside their own 22, eventually clearing to touch but, from the resulting lineout the Ospreys struck, taking the lead for the first time with the first try of the afternoon.
The ball was moved into midfield from the setpiece where it went to ground off the back of the Scarlets’ Jonathan Davies in the tackle. It was Luke Morgan who reacted first, hacking the loose ball forward and winning the race to ground it, the try confirmed by the TMO.
Davies added the conversion, and then brought the hour up just a minute later with a drop goal from in front of the posts to take the scoreboard to 16-9.
Davies was having a big influence on the contest and he took the Ospreys 10 clear with 13 minutes to play with his second drop goal after the forwards had done the hard work at close contact.
As the clock moved into the final 10, a call from touch judge, Nigel Owens, saw the TMO intervene and award a penalty against Gareth Thomas for a no-arms tackle, Dan Jones putting over the three points.
The Scarlets piled on the pressure in the final minutes, looking to steal a share of the spoils, but with the Ospreys defence looking imperious it seemed all over when Owen Watkin won turnover ball under his own posts after 12 phases of pressure, with 90 seconds remaining.
The home team looked to run down the clock but confusion reigned when the crowd counted down the final 10 seconds past the 80 minutes for Sam Davies to put the ball out of play.
Referee Ben Whitehouse ruled that there was still three seconds remaining, handing the Scarlets a throw on the Ospreys line for the final play. Thankfully, Tipuric was able to spoil at the front of the lineout and although Ryan Elias gathered the loose ball he was bundled into touch by Alun Wyn Jones and Gareth Thomas, sparking a mass brawl on the touchline and wild celebrations in the stand as the full-time whistle blew belatedly.