The pair came on for Michael Dawson and Aaron Lennon in the 70th minute and just over 10 minutes, both had scored to clinch the 2-0 win and maximum points.
It was a testing afternoon as sleet and snow whipped around the Lane with Pompey's 4-1-4-1 system often seeing nine men behind the ball and clear-cut chances at a premium.
Dawson rattled the crossbar in the first half, Paul Robinson saved superbly from Sean Davis and Didier Zokora hacked off the line from Kanu as we dictated possession but simply could not carve the visitors open.
That all changed with 10 minutes left when Dimitar Berbatov volleyed into the floor and Bent planted home a header from 10 yards.
Two minutes later and Bent turned provider, beating Hermann Hreidarsson down the left and squaring for O'Hara to side-foot into the corner.
"They did well when they came on, scored the goals and both contributed well to the team," reflected Juande afterwards.
"We took a centre-half off for a forward, it's a risk but it worked this time.
"The decisions to make changes are not made for the sake of it. It is to turn matches around. They have worked well. The players are aware of why we make them - a solution for a game that is in stalemate or blocked up."
Juande Ramos hailed the match-winning contribution of substitutes Darren Bent and Jamie O'Hara after the 2-0 win against Portsmouth on Saturday.