Beckett scored 18 goals on loan at Oldham last season and was gunning for goals on his return to the Huddersfield side.
But Town's home form has been woeful this season and there was to be no joy for Beckett.
This was Oldham's fourth successive victory and they look a solid bet for the play-offs.
Porter grabbed the clincher just before the hour mark between goals from Gary McDonald and Paul Warne.
At 3-0 down Beckett was taunted by the away fans who chanted: "Beckett, Beckett. What's the score?" in his direction.
The striker responded with four chances - including two in two minutes - but there was no way back and Huddersfield went down to their fourth defeat in the last five home games.
Hundreds of Huddersfield fans walked out long before the end and those that remained booed the players off the field and there were angry protests outside.
Oldham were in charge throughout and after Porter headed over Andy Liddell's free-kick in the fourth minute it was one-way traffic.
A McDonald volley left Town keeper Matt Glennon rooted to the spot but fortunately for him it went over the top.
McDonald put the Latics ahead in the 16th minute when the impressive Chris Taylor burst away down the left and fired a curling effort across goal.
Glennon scrambled over and palmed it out but the ball fell perfectly for McDonald to finish from a tight angle.
Chris Brandon, back from injury, looked dangerous for Huddersfield but he tired as the game went on and defender Sean Gregan, on loan from Leeds, had a battle royale with Andy Booth.
Taylor almost doubled the lead with a brilliant run. As defenders stood off him he powered forward but his measured shot curled just inches wide of the far post.
Oldham turned the screw after the break and Taylor did the damage again beating Andy Holdsworth for pace and setting up Porter for a side-foot finish for his 11th goal of the season in the 59th minute.
Eight minutes later it was all over when Warne added the third via a deflection off a defender which left Glennon stranded.
That was the cue for the Latics fans to have a go at Beckett but he hit back with two chances in two minutes, the first a superb flick header which would have gone in on any other day.
As Huddersfield pumped balls forward to Beckett and Booth there were chances aplenty but it was just not Town's day.
Porter should have completed the rout six minutes from time when Glennon misjudged a cross but the striker planted his header wide.