Cardiff crashed to a disappointing derby defeat as the Ospreys bludgeoned their way to a comfortable United Rugby Championship victory.
The Blue and Blacks took a slender 3-0 lead into the interval following a tense and error-strewn first half.
They soon stretched their lead but were undone as the Ospreys powered to three tries in ten minutes through Dewi Lake, Rhys Webb and Sam Parry.
It was a shaky start for Cardiff, who were perhaps harshly penalised shortly after kick-off when Liam Belcher attempted what appeared to be a legitimate jackal. The Ospreys kicked to the corner and attempted to get their driving line-out going.
They rumbled towards the whitewash and it appeared as though Dewi Lake had scored inside two minutes but Gianluca Gnecchi’s on field decision was ball held up and television replays gave no means to overturn the decision and he actually awarded Cardiff a penalty.
Cardiff cleared but remained under severe pressure as the Ospreys dominate the opening exchanges. An over-ambitious interception attempt saw Owen Lane sent to the sin-bin and the Ospreys went to the corner once more.
Their driving lineout would undoubtedly have been a hot topic of discussion and preparations all week at the Arms Park and it was once again repelled.
The Blue and Blacks remained stuck in their own half but the tide began to turn when the pack earned a penalty with a dominant scrum. It gave them a brief reprieve but when the lineout misfired, alarm bells rang.
Rhys Webb snaffled the ball at the back and chipped an intelligent kick to the corner. With Lane in the bin there was plenty of room and Keelan Giles was unopposed as he raced towards the ball but with the try-line at his mercy, he failed to regather.
It was an incredibly tense and scrappy opening quarter before Priestland finally opened the scoring with a well-struck penalty on 27 minutes.
The mistakes continued and the Ospreys piled on the pressure on the half-hour mark before a humongous tackle from Rhys Carré on former teammate and Arms Park favourite Alex Cuthbert brought the attack to a brutal end.
From there, virtually on their own try-line Cardiff went on the attack with Priestland and Lane combining down the left. They did not make it out of their 22 but a barnstorming break from James Ratti did.
The number eight, playing against his former side, powered through Alun Wyn Jones and Will Griffiths as he roared into the Ospreys half. He looked to put Tomos Williams away but the pass went behind the scrum-half.
Williams still managed to collect the ball at second attempt and maintained momentum but his speculative offload failed to find Lane on the right.
In the closing stages of the half, Michael Collins thought he had scored but again Gnecchi’s on field decision was no-try and replays showed he was well short and lost possession.
Cardiff took their slender 3-0 lead into the break and Priestland quickly doubled it within three minutes of the restart.
Former Cardiff fly-half Gareth Anscombe soon responded with a long-range effort, which crept over the crossbar to bring the scores to 6-3 with 35 minutes remaining.
Ten minutes later the Ospreys took the lead as Anscombe kicked to the corner and their lineout eventually rumbled over with Lake the beneficiary. Anscombe converted to give his side a four point lead.
Cardiff were dealt a major blow as they went through a succession of phases before Rhys Webb intercepted Rhys Priestland’s pass and went the length.
Anscombe knocked over the extras and the Ospreys were now in a commanding position with a 6-17 lead.
It appeared ominous for the home side when it appeared that Cuthbert had created another quick-fire try for the Swansea club but replays showed he knocked on the bouncing ball before Webb finished.
There was a collective sigh of relief at the Arms Park but it did not last long as the penalties continued and a clever set move from the lineout saw Sam Parry cross.
Finally, for the first time this season, Anscombe was off target but Cardiff faced an uphill battle, trailing 6-22.