The striker headed home the opener before taking advantage of some comical goalkeeping from Johny Placide to double the advantage.
With snow falling throughout the game, there were fears that it might be abandoned when play was stopped during the second half.
But it started up again and the Blues saw out the closing stages with relative ease, claiming a clean sheet to complement their three-point haul.
Kenny Jackett made two changes from the side beaten at home by Gillingham the previous week.
Pitman returned from a three-game injury absence to skipper the side, while Brandon Haunstrup also started, as Christian Burgess and Sylvain Deslandes dropped to the bench.
The snow had been falling heavily before kick-off, but bright sunshine had taken over as the sides emerged from the tunnel.
It would not be the end of the white stuff, however, with plenty more flurries following throughout the afternoon.
There were not an abundance of chances in the opening stages, although both sides looked to get forward whenever possible.
Anton Walkes’ failure to deal with a Cameron Dummigan delivery caused a few butterflies, but the right-back recovered well to clear the danger.
Placide was also giving the home support reason to be nervous, with the keeper dallying in possession on several occasions.
One instance saw Connor Ronan tackle him, only for the ball to land clear and let the maverick stopper off the hook.
A quick break presented Oldham with their best chance of the half, only for former Fratton loanee Eoin Doyle to stab his shot straight at Luke McGee.
Jamal Lowe soon had an opportunity at the other end, bursting between two defenders and seeing an effort deflected wide.
And they went even closer soon after, with Matt Clarke sending Dion Donohue’s corner into the mixer, where Jack Whatmough’s header was cleared from the line by Wilfried Moimbe.
But the deadlock was broken on 29 minutes, as PITMAN rose to meet Ronan’s cross and send a deft header past Placide and into the net.
Pompey’s skipper was also proving useful at the other end of the pitch, getting back to nod clear a corner.
The hardy travelling support were then given more reason to cheer when the Blues doubled their advantage before the break.
Placide had failed to learn the error of his ways, with PITMAN stealing the ball from his feet and slotting home from an acute angle.
Oldham Athletic 0
The Latics were given an opportunity to get back into the contest straight after the restart, when Clarke was booked for a foul on Craig Davies.
Kean Bryan took the free-kick from just outside the Blues box and although his effort went through the wall, it was straight into the arms of McGee.
It was becoming to difficult to spot Pompey’s white-shirted players, as the snow fell more heavily – and began to lay.
There would have been a few worried fans in the Zen Office Stand when referee Sebastian Stockbridge called a halt to proceedings.
“Off, off, off,” cried the Oldham supporters, perhaps in hope more than expectation and – following an eight-minute delay while the lines were cleared – the action resumed.
Doyle then had a couple of shots saved by McGee as the hosts tried to find a route back into the contest.
The second was closest to finding the net, with Pompey’s keeper just about getting his fingertips to it and diverting the ball over.
And McGee also dealt with a low drive from substitute Duckens Nazon, as time started to run out for Oldham.
But the Blues almost wrapped things up on 85 minutes when Pitman teed-up Kal Naismith, only for Placide to put the forward off, with Dummigan hacking the ball clear.
The earlier delay meant 11 minutes of stoppage-time were displayed by fourth official Ken Haycock, giving hope to the home support.
But it was Pompey who almost added to the scoring when Lowe cut inside from the right and drilled a low shot against the inside of the post.
Oldham (4-4-2): Placide; Dummigan, Wilson, Bryan, Moimbe (Nazon 80); Byrne, Fane, Gardner, McLaughlin (McEleney 55); Davies (c) (Amadi-Holloway 59), Doyle
Subs not used: de la Paz, Hunt, Edmundson, Pringle
Pompey (4-1-4-1): McGee; Walkes, Whatmough, Clarke, Haunstrup; Close; Lowe, Donohue, Ronan, Naismith; Pitman (c)
Goals: Pitman 29, 42
Booked: Clarke, Pitman, Donohue
Subs not used: Bass, Burgess, Deslandes, Widdrington, May, Kennedy, Chaplin
Referee: Sebastian Stockbridge
Attendance: 4,306 (767 Pompey fans)