REEBOK REGIONAL ROUND UP U20
The Blues, without a win this season, led 26-10 with 10 minutes remaining thanks to tries from James Connors (2) and Matthew Edwards and 11 points from the boot of Treorchy outside half Celyn Ashton.
However, the Ospreys, who went in to the game with a 15 point advantage over the third placed Dragons struck back with three converted tries.
Neil White and Jason Howells had already bagged a try for the Ospreys before Anthony Rogers, Ross Cridland and Jonathan Spratt raced over for the table toppers. Aaron Bramwell added three conversions to make doubly sure of a crucial victory.
The game between the Dragons and Scarlets was postponed and will now be played next Wednesday.