The defender rose above the Forest defence to plant a firm low header into the corner of the net.
And although Forest keeper Rune Pedersen appeared to make the save, the ball, with some help from David Johnson, trickled over the line.
Earlier in the afternoon Swindon had taken the lead when New Zealand international Rory Fallon scored on 31 minutes.
Forest soon hit back when impressive young winger Kris Commons snuck around the back of the Swindon defence to equalise.
Going into the game Swindon where able to call upon Fallon as the striker had completed a two-match suspension.
And he lined up alongside Jamie Cureton to spearhead the Swindon attack for the first time this season.
The Robins also included fans' favourite Nicky Summerbee as the veteran winger returned to the County Ground after an 11-year absence.
It was the home side who started the brightest with Fallon strong in the air and Cureton busy around the penalty area.
The former Reading and Bristol Rovers man had two good opportunities in the first 20 minutes but unfortunately on both occasions fired wide.
But he soon atoned for these misses when he dissected the Forest defence for strike-partner Fallon to finish well past Perdeson despite team mate Nicky Nicolau nearly getting in his way.
The home side were the victims of their own downfall seven minutes later when a speculative cross from Forest midfielder Nicky Southall was not dealt with by Patrick Collins and Commons struck back.
The conditions seemed to undermine both teams in the second half as a number of passes went astray.
But it was the home side who perhaps had the most self-belief and this was emphasised by O'Hanlon's winner on 73 minutes.