Joshua Jones bagged a first half penalty for the North, who followed up successive victories in the opening weeks to assume top spot in the championship table. The Ospreys, who won all three titles last season, looked a confident unit in the first half and were deserving of their lead thanks to Rees's try and a conversion from the highly rated pivot Phillips.
"They took a first half lead, but after that we were dominant and probably deserved to win it in the end," said Blues High Performance Manager, Gafyn Cooper. "Our forwards came together after the interval and that was crucial to the game."
In the other game, the Dragons registered their first win of the season, beating the Blues South 10-0 at Pandy Park, Cross Keys.
Having come close without winning either of their opening games, the Dragons took the victory spoils thanks to a try from No.8 and Captain Craig Attwell and a conversion and penalty from full back Joseph Evans.
Leading by the final score at the interval, the Dragons had to withstand some heavy pressure from the South in the second half. Their plight was made even more difficult when first Craig Howell and then Greg Baker were yellow carded. However, they held on and managed to pick up a first win the new campaign.
High Performance Manager Ron Schottle said: "It's a relief in a way, but I thought we deserved to win the game despite some real pressure in the second half. I would like to think we can go on from here."
Tuesday night's scheduled game at U20 level between the Ospreys and Dragons will now be played next Tuesday.