Jarrod Evans fell agonisingly short with the last kick of the game as Ospreys clinched a nail-biting win at Cardiff.

Josh Adams' try looked to have rescued a draw for the hosts in a compelling if not thrilling derby at a packed-out Arms Park.

But Cardiff indiscipline proved costly amid an Ospreys second-half comeback.

Cardiff looked on course for a fourth win in a row when they led 12-7 at half-time through tries from Owen Lane and Liam Belcher.

However, Ospreys hookers Sam Parry and Dewi Lake scored identical driving lineout tries, either side of an opportunist score from Williams, as the visitors claimed a fifth successive win over Cardiff.

A sell-out crowd of 12,125 then held its breath after Jack Walsh was penalised in injury time, allowing Evans a final attempt to level for the second time from close to the touch line - only to fall inches short.

It marks a third win in a row for Ospreys who are slowly starting to piece their season back together.

It had looked a different story early on when another Williams - Wales star Liam - took just five minutes to show the home crowd just what they have been missing.

Injured on his debut back in September, the full-back created the move that led to the game's opening try on his return.

Williams comfortably won a high ball inside Ospreys territory before Tomos Williams poked through a devilishly awkward kick. The bounce eluded Walsh who could only watch on in horror as Lane was left with an easy first league try of the season.

Ospreys shook off the early blow and clearly fancied their chances up front.

Ospreys scrum dominance

Their scrum, anchored by Tom Botha, had the edge enough to have Cardiff's Taulupe Faletau constantly on the back foot at number eight.

And after their first meaningful lineout drive was rebuffed, a second saw Parry dive over on the back of Gareth Thomas' efforts. Fly-half Williams defied the torrential rain to level the scores.

Not to be outdone, Cardiff's pack hit back with a drive of their own within six minutes - and again it stemmed from Liam Williams.

His clever cross-field kick found Adams in space and from the resulting penalty, Belcher barged over to regain the lead.

Having both scored tries, both hookers were then forced off within the space of three minutes before half-time as a full-blooded derby took its toll.

Trailing 12-7 at the break, Ospreys turned the tables on their hosts with two tries to lead 19-12.

Cardiff again did well to defend a lineout but again the scrum was their Achilles heel.

Scrum-half Williams was snagged by Jac Morgan as he kicked clear, Adam Beard tipped the ball that fell into the grateful hands of Owen Williams who dived over from a few metres out.

Referee Adam Jones overruled the television match official to award the score despite Cardiff claims of blocking.

Cardiff's indiscipline continued to gift Ospreys easy territory and replacement hooker Lake, returning from an injury in October, touched down from another lineout to lead 19-12.

The game looked to have tipped in Cardiff's favour when Ospreys number eight Ethan Roots was shown a yellow card for his illegal tackle on Liam Williams.

It looked a close call but it would make a big impact moments later when Cardiff stole a scrum against the under-strength Ospreys pack.

Evans was alive to the opportunity and his cross-field kick sent Adams racing over from 30 metres out - his seventh try in six games - and Evans' conversion levelled the scores at 19-19.

Ospreys' Williams missed a long-range penalty but was handed another chance after Kirby Myhill was sent to the sin bin for a high tackle and he took it.

Ospreys head coach Toby Booth:

"In conditions like that then the fundamentals of rugby take over like set-piece and defence, which are things that we're known for.

"But you also have to deliver and the boys did that. Rugby is a collision game. Sometimes you just have to go through the front door and we dominated that area.

"To back up a derby win is special. We played the second half on our terms and in tough games its about who can be the smartest."

Cardiff head coach Dai Young:

"It's always disappointing to lose. There's not between all four Welsh regions and that was another to go down to the wire.

"The set-piece was a factor but so were the penalties in the second half.

"We struggled to get out of our half with our indiscipline and we made poor decisions around the breakdown which gave them easy field position and momentum. If you keep giving the Ospreys lineouts five metres out, they're going to convert a few of them."

Cardiff: Liam Williams; Owen Lane, Rey Lee-Lo, Ben Thomas, Josh Adams; Jarrod Evans, Tomos Williams; Rhys Carre, Liam Belcher, Will Davies-King, Lopeti Timani, Seb Davies, Josh Turnbull (capt), Thomas Young, Taulupe Faletau.

Replacements: Kirby Myhill, Corey Domachowski, Keiron Assiratti, Teddy Williams, Ellis Jenkins, James Ratti, Lloyd Williams, Mason Grady.

Ospreys: Jack Walsh; Alex Cuthbert, Michael Collins, Keiran Williams, Keelan Giles; Owen Williams, Rhys Webb; Gareth Thomas, Sam Parry, Tom Botha, Adam Beard, Alun Wyn Jones, Jac Morgan, Justin Tipuric (capt), Ethan Roots.

Replacements: Dewi Lake, Nicky Smith, Tomas Francis, Rhys Davies, Morgan Morris. Reuben Morgan Williams, Joe Hawkins, Cai Evans.

Referee: Adam Jones (WRU)

Assistant referees: Ben Breakspear and Carwyn Williams (WRU)

TMO: Wayne Davies (WRU)