Outside half Josh White, who had earlier kicked a penalty, converted from the touchline and the Ospreys managed to hold out for the final few, heart-stopping moments.
Jarrod Evans' 25th minute penalty provided the North with the advantage and when scrum half Shaun Clarke muscled his way in for an unconverted try shortly after half time, the visitors looked well set for a fourth successive victory of the campaign, and a first away from their home base at Abercwmboi.
However, it was the Ospreys who came out on top thanks to Popham and White with team manager Graham Court admitting that his side let out a huge sigh of relief on the final whistle.
"It was an excellent game once again but a very tight one at that," said Court. "To be honest the North had plenty of chances but I thought we defended particularly well and hung in there until Liam grabbed his chance. The conversion was a tough one, but Josh has been magnificent for us this season and he made no mistake. We were glad to hear the final whistle."
There was equal joy at Parc y Scarlets where the Scarlets managed a first win of the season, 14-7 against the Dragons. A try from Ashton Hewitt, converted by Lloyd Rogers put the Dragons in front during the first quarter, but by half time, the hosts were level courtesy of a try from scrum half Gareth James and a conversion from Jack Maynard.
The issue was settled a minute into the second half when Bradley Davies was driven over for a try that Maynard improved. "It's just a relief to get a first win of the season," said Scarlets team manager Wyn Thomas. "We believe we have a decent side here, but to be honest it has taken us a little while to get going. Hopefully we can kick on from this."
Not such a close run affair at Cardiff Arms Park where the Blues U18s ran out 41-7 winners against the Ospreys, who have won only once in three outings this term.
Cory Allen, Harry Robinson (2), Owen Rees (2) and Ryan Evans all crossed for the Blues with outside half Rhys Patchell adding three conversions and Garin Thomas one. Patchell also kicked a penalty while for the Ospreys, Nicky Davies scored their lone try and Sam Davies converted. Among the interested spectators at the game were Blues director of rugby Dai Young, Scarlets coach Nigel Davies, father of Ospreys' outside half Sam and Ospreys' director of rugby Andrew Hore.
Down the road at Newbridge, the Dragons came close to ending the Scarlets unbeaten record - eventually going down 26-20 to a side inspired, once more, by outside half Jordan Williams.
Having scored 36 points in his side's opening two victories, over the Ospreys and Blues, Williams helped himself to another 21 as the visitors edged out a combative Dragons side.
Wayne Pugh, Dragons team manager said; "It was a decent performance although at times, we did lack a bit of discipline. I think Jordan Williams looks a really decent lad and he was the difference to night."
Williams scored a try and kicked a conversion and four penalties while replacement scrum half Rhodri Williams added a second try for good measure. For the Dragons, who trailed 13-8 at the interval, there were tries from Dominic Franchi, after 10 minutes, Elliot Dee after 44 and replacement hooker Jack Williams 10 minutes from time. Ethan Davies kicked a penalty and a conversion, but it was not enough to deny the Scarlets a third successive win of the season