Jordan Bishop was the lone scorer for Blues South in their game at Rumney RFC - the young prop crossing for a second half try after the Ospreys had swept into a 10-0 lead courtesy of unconverted tries from scrum half Daniel Brooks and leading scorer, wing Dafydd Howells.
A third try, from Jack Anderson, sealed the Blues' fate with Ospreys team manager Graham Court saying; "It was a good performance and a very good game too. The South tested us throughout, so it was good to come out of it with another victory."
Down the M4 in Llanelli, the Scarlets under 16s recovered their poise to beat a Blues North side that had been beaten only once in their opening seven games this season.
Tom Samuel and Sam Edwards put the visitors in the driving seat with early tries but Steffan Evans replied with a converted try before the interval to leave the Scarlets within seven points.
A Tom Phillips try and Jack Maynard conversion brought the sides level before the North forged ahead once again with a try from the outstanding back rower Jack James. However, a penalty from Maynard took the Scarlets to within four and a converted try from Craig Woodall won the day.
Scarlets team manager Wyn Thomas said, "Credit has to go to two sides who were determined to make this a really decent spectacle in desperately cold conditions. It was a real advert for the competition and I'm obviously delighted that we not only took part in such a game, but that we won it too."
Maynard finished with 12 points from three conversions and two penalties, whilst North outside half Sam Withers helped himself to a penalty and three conversions.
The race for a place in the U18 final is even more intriguing after the game between leaders Blues and third placed Scarlets, at Abercwmboi, finished 16-16.
Calum Lewis scored the Blues' only try with leading points scorer, outside half Rhys Patchell, adding three penalties and a conversion. However, the Scarlets were not to be outdone and lock Dyfed Morgan's try and a conversion and three penalties from full back and captain Luke Treharne was enough to earn a share of the spoils.
The Blues are now five points clear of the Ospreys and Scarlets who are locked together in second, on 18 points. Next week's games see the Ospreys and Blues meet at Llandarcy and the Scarlets take on the Dragons. The two games are likely to have a significant bearing on who will qualify for the final.
The Scarlets wind up their pool campaign with a trip to North Wales to play RGC 1404, whilst the final game of the regular season sees the Ospreys go to the Dragons.