The Lasses conceded for the first time under the Australian manager, but bounced back with six minutes left to play, forcing an own goal from the home side to ensure they went level on points with third-placed Bristol City Women.
After being pegged back in the second half and seeing a Pompey side grow in confidence, Oxtoby watched on with pride from the sidelines as she saw new things from her team to get them over the line.
"We knew coming here was going to be a difficult place to come," Oxtoby said post-match. "We knew there were going to be elements of the game we'd control and other elements when we'd have to dig in and fight, but it's just important to leave with three points.
"I think the reaction from us to make sure we stayed in the game (after being drawn level) and force errors of our own shows the character of this group.
"I'm so proud of the professionalism that the girls showed today because it would've been easy to get drawn into the theatrics of it all.
"I think from our perspective, the girls did really well to hold their ground but not get drawn into anything silly."
United extended their unbeaten league run to seven games, having not tasted defeat since the end of September, to put themselves in the mix with the top three after achieving Oxtoby's points tally target for this point in the season.
With 11 league games coming in 2026, starting with Birmingham City Women, Newcastle have a big second half of the season awaiting them.
"We needed to stay in touch with those above us, and the aim was 19 points going into the Christmas break, and we've done that now. So we need to rest, recuperate and get ready for coming back in the new year.
"It's going to be a big second half of the season for us, so we'll all go away, recharge our batteries and come back in 2026 fresh and ready to go."
November's Sela Player of the Month, Jordan Nobbs, has now got two goals in her last two WSL2 starts, which have proved to be critical moments in a pair of victories.
It was Nobbs' forward run which forced Portsmouth into a late error and handed Newcastle a lead in the second half, but she's still doing things for Newcastle that are going under the radar, Oxtoby explained.
"Hopefully, Jordan is enjoying her football, and it's showing in the way that she's playing," added the Magpies manager. "Everyone talks about what she does with the ball, but it's what she does without it as well.
"She can instigate a press and just makes good decisions on the pitch. It's a pleasure to get to coach her every day, and hopefully she's enjoying her football with us too."
