The first half saw the O's take the lead four minutes before the break when Tom Morgan pulled the ball back for David Stone to smash into the roof of the net from the edge of the box.
Portishead equalised through Calum Townsend on 55 minutes but straight from the kick off, good work from Ryan Dunn saw Josh Williams net to regain the lead for Oldland.
With fifteen minutes left, a mix up in the home defence saw Morgan seize on a back pass to fire home his ninth goal of the campaign to seal all three points for the O's.
They now host Wincanton Town on Wednesday night, kick off at 7.45pm.