Coach Paul Gerrard told the Oldham players they had passed a "massive test" after overcoming Portsmouth 1-0 at Boundary Park.
Jose Baxter's early goal was enough to sink rock-bottom Pompey, who had Sam Sodje sent off for violent conduct just after half-time. The Latics warmed up for Tuesday night's FA Cup fifth-round replay at Everton with their third straight league win since parting company with manager Paul Dickov.
Baxter's 11th-minute strike, which beat Portsmouth goalkeeper Simon Eastwood from 25 yards out, lifted Oldham further away from the League One drop zone.
Gerrard, assistant to caretaker boss Tony Philliskirk, said: "It was a scrappy game but a good result. We kept telling the players they shouldn't underestimate Portsmouth because this was a massive test, so their response was good.
"When you win a couple of games there can be a tendency for unprofessionalism to creep in, but they got the job done. We had chances to score a couple more and our defending was excellent when it needed to be.
"In our league position, and in our predicament, it's points that matter more than performances. We weren't at our best, but we are going into games with a bit of confidence. It's Everton next and we can go there and believe in our chances of getting a result. Obviously they are big favourites at home, but stranger things have happened."
Portsmouth manager Guy Whittingham bemoaned his team's early mistakes as their depressing run without a win reached 22 games.
He said: "We knew Oldham would cause us a problem if we didn't pressure them, but that's what happened. It was only after they scored that the players started doing what they were supposed to do, so the slow start has cost us. Our keeper has made a mistake and would usually stop that shot, but Simon (Eastwood) has done well for us this season and we move on."
Whittingham revealed that Sodje had apologised to his team-mates for the off-the-ball clash that earned him a red card.
"He has come in and said sorry, but that's no good to us," Whittingham said. "There was a bit of an incident and everyone got excited, but I genuinely didn't see what happened. I'll have to look at the DVD and see it for myself."