The 33-year-old cracked a first-half hat-trick, including Hereford's 1,000th League goal as the Bulls went on to demolish the League One promotion hopefuls.
No less dominant was on-loan winger Lionel Ainsworth, who set up two of Guinan's goals as well as getting two of his own.
Guinan was in irresistible form in the first period as he and Ainsworth gave the Latics defence a torrid time.
The veteran striker opened the scoring after just three minutes when his flicked header from a Matt Done throw beat keeper Greg Fleming.
Guinan was on target again in the tenth minute when he slid in an Ainsworth cross from close range.
In between, Windass saw a shot clip the outside of the post, but Hereford were not in any mood to ease back and extended their lead on 26 minutes.
Guinan's header from an Andy Williams cross came back off the crossbar and Ainsworth hammered in the rebound.
If Guinan himself was unfortunate there, he only had to wait until the 41st minute to complete his treble, sidefooting in another cross from the lively Ainsworth.
In the second period, Oldham, was expected, pressed forward to try to get some consolation from the game.
But it was Hereford who scored again in the 62nd minute, with Ainsworth stealing the ball away from the hapless Kelvin Lomax before racing on to beat Fleming in a one-on-one situation.
After that, although Bulls keeper Craig Samson made a handful of saves from long-range efforts, it was the home side who looked the more likely to extend their tally.
On a more fortunate day, Williams would have certainly converted one of a number of chances which came his way.