United Rugby Championship
Cardiff (5) 13
Tries: Lloyd, Hughes Pen: Thomas
Ospreys (3) 13
Try: Edwards Con: Edwards Pens: Edwards 2
Ospreys' revival under Mark Jones continued after claiming a well-earned draw at high-flyers Cardiff despite a red card.
Jones claimed a late win over Scarlets in his first game since succeeding Toby Booth as head coach.
And Ospreys came within inches of clinching another dramatic victory even after James Fender's first-half sending off.
Evan Lloyd gave Cardiff a half-time lead in a chaotic and disjointed derby.
But a gutsy defensive display, a dominant scrum and 13 points from fly-half Dan Edwards ensured Ospreys left a sold-out Arms Park on level terms.
It could have been more but replacements Lewis Lloyd and Morgan Morse both saw tries ruled out in a nerve-tingling climax.
Jac Morgan, Sam Parry and Rhys Davies, on his first appearance since March, weighed in with tireless displays.
But Cardiff will kick themselves that they did not claim their first victory against Ospreys here at the Arms Park since 2007 having led 5-3 at half-time against 14 men.
Indiscipline proved costly as yellow cards for Teddy Williams and Rhys Litterick meant they spent half of the second period also down in numbers.
But the stop-start nature of the game also played into the hands of the Ospreys' set-piece.
Cardiff began the game with an early try from their driving lineout and ended the half again on the front foot.
But the 30 minutes in between was a tale of Ospreys failing to capitalise on their scrum dominance.
Reuben Morgan-Williams burst 40 metres clear after catching opposite number Ellis Bevan horribly out of position but Morgan Morris spilt possession over the line.
Edwards missed a penalty, Max Nagy was stopped on the line by Louie Hennessey while the full-back's knock-on would eventually rule out Morgan-Williams' try.
It was an eventful debut for Hennessey – on loan from Bath – who began at centre, won a lineout in the build-up to Lloyd's try, saved a score and ended on the wing after injuries.
Teddy Williams, who impressed alongside the abrasive Seb Davies, stole a crucial lineout before his opposite number was sent off a minute before half-time.
Fender was guilty of a "high degree of danger" according to referee Ben Breakspear when he rolled Dan Thomas by the neck while applying weight on the Cardiff flanker's lower leg.
Ospreys boss Jones gave up Nagy at half-time in order to keep a full-strength pack, through replacement James Ratti.
And he was rewarded with the lead early in the second half.
Cardiff lock Williams was shown a yellow for a lineout infringement on 49 minutes and Edwards swiftly made the most if it with a try out of nothing.
The 21-year-old looked well marked eight metres out but stepped inside Rory Jennings and shrugged off Callum Sheedy as he powered over.
Matt Sherratt had opted for six forwards on his bench and threw on five of them in one go to provide the fresh legs to power over from another lineout drive to level the scores.
Edwards and Thomas traded penalties but it was Ospreys who rallied late, camped on the Cardiff line but - again - could not make the moment count.
Cardiff head coach Matt Sherratt said: "It's a difficult one to judge. I really thought we would kick on after the sending off and get a bonus point in the second half. So it feels like two points dropped.
"But we gave away far too many penalties so the game meandered from one set-piece to another and got away from us.
"When you consider we held them up over the line at the end, then we're pleased with the draw."
Ospreys head coach Mark Jones said: "I'm very happy with the fight and the quality the players showed after the red card but we're all frustrated with the result.
"We didn't take our opportunities in the game, especially right at the end. We have nailed those moments in the past but we didn't today.
"We had to reorganise after the red but we felt keeping a full pack was the way we could win the game. It was a risk but I think it paid off."
Cardiff: Cam Winnett; Josh Adams, Louie Hennessey, Ben Thomas (capt), Gabriel Hamer-Webb, Callum Sheedy, Ellis Bevan; Danny Southworth, Evan Lloyd, Kieron Assiratti, Seb Davies, Teddy Williams, James Botham, Dan Thomas, Alun Lawrence.
Replacements: Daf Hughes, Corey Domachowski, Rhys Litterick, Josh McNally, Thomas Young, Taulupe Faletau, Aled Davies, Rory Jennings.
Sin bin: Williams (49), Litterick (68)
Ospreys: Max Nagy; Iestyn Hopkins, Owen Watkin, Keiran Williams, Keelan Giles; Dan Edwards, Reuben Morgan-Williams; Gareth Thomas, Sam Parry, Tom Botha, Rhys Davies, James Fender, Jac Morgan (capt), Justin Tipuric, Morgan Morris
Replacements: Lewis Lloyd, Garyn Phillips, Rhys Henry, James Ratti, Morgan Morse, Kieran Hardy, Jack Walsh, Evardi Boshoff.
Red card: Fender (39)
Referee: Ben Breakspear (WRU)
Assistant referees: Ben Connor &Adam Jones (WRU)
TMO: Tom Spurrier (WRU)