Blues South under 16s took the honours on derby night as the Blues North suffered a fourth defeat of the season in the WRU Age Grade Championship.
Blues North, who defeated the Ospreys at Llandarcy last time out, failed to bag their second win of the season despite a Jack Barratt try and eight points from the educated boot of centre Callum Phillips.
Still, the night belonged to South wing Osian Davies who swept in for a try hat trick. Gareth Thompson added 10 points with the boot to make it four wins in six games.
The two sides were well matched for long periods of the game, but Davies took his opportunities well and that was the difference.
The Scarlets under 16s edged past the Ospreys to stay on top of the pile.
However, the hosts were made to work for their 17-13 success whilst in the other game, Blues South defeated Blues North 25-13 at Taffs Well.
Although they trailed 13-12 at half time, the Scarlets won the day with a single try in the second half from wing Steffan Evans.
Before the interval, Jake Randall and Gethin Davies crossed for the Scarlets with leading scorer Jordan Williams adding a conversion for good measure.
However, it was the Ospreys who led at half way courtesy of tries from Sam M Davies and Rhys Williams and a Sam Davies penalty.
Scarlets team manager Wyn Thomas said; "It was an excellent game and could have gone either way.
"We trailed narrowly at half time but Steffan Evans' try put us back in front two minutes into the half. After that, the Ospreys pressed hard, but our defence was good and for that alone we deserved to hold on."
Supporters are urged not to delay and join us for our big Boxing Day fixture with Newport Gwent Dragons in the Guinness PRO12 - with tickets selling fast for the huge game at BT Sport Cardiff Arms Park!
Supporters who have purchased tickets and are travelling to the European Rugby Challenge Cup clash with London Irish at the Madejski Stadium on Saturday (kick-off 3pm) are advised that some seating has now been changed for this fixture.
Supporters are advised that we will selling tickets right up until kick-off for the big home European Rugby Challenge Cup clash with London Irish - so don't miss all the action this Saturday at BT Sport Cardiff Arms Park.
Blues Rugby Camps will be back this February to given children right across our region the opportunity to participate in rugby during the holidays - and you can book NOW to avoid any disappointment and get the ideal Christmas gift for the aspiring young rugby player in your life!