Scarlets centre Johnathan Davies scored the U20's try and there were penalties from Cardiff wing Leigh Halfpenny and Swansea outside half Dan Bigger. But the Ospreys side, containing 10 of the players who faced Leinster recently, ran in four tries in their 26-11 triumph at Aberavon.
David Bishop, Aled Brew, Kristian Phillips and Jonny Vaughton crossed for the Ospreys and there were two conversions from Shaun Conner and a third from Phillips.
"This was just the kind of workout we needed. It was hugely physical and highly competitive," said Wales U20 Team Manager Bob Beale.
"We have a big game to look forward to against England at Gloucester on February 1st and this was an ideal build up towards that game."