Oldham conceded five goals on the final day of the season, despite taking the lead early on, to end on three straight defeats for 2018/19.
Head Coach Pete Wild made four changes to the side that played at Newport in the week. With Iversen returning between the sticks whilst Edmundson and Clarke missed out completely through injury.
There was also a proud moment for 19 year-old Tom Hamer who was handed the Latics armband, with Javid Swaby-Neavin making the bench after signing professional forms.
On a chilly afternoon the first meaningful bit of action came via a goal courtesy of Johan Branger-Engone on the tenth minute.
A maurading run by Lang culminating in a one-two with O'Grady, meant the ball eventually found Branger who hit it first time, connecting with the left-side post before nestling into the net.
Latics conitnued to dominate early on with the next chance coming seven minutes later. A free-kick on the right was whipped in by Maouche, Sylla's glancing header going wide of the Cobbers goal.
With not much action inbetween, Northampton equlised in emphatic fashion on the 21st minute with the first shot on goal. A ball pumped forward fell to Hoskins on the edge of the box unleashing a fantastic effort that flew into the top corner.
Keith Curle's men started to get a foothold with Foley making Iversen produce a good save before their captain Aaron Pierre nodded home from the resulting corner on the half-hour mark to put the visitors in front and complete the early comeback.
It got worse for the home side just before the interval. Williams finishing the move with a tap-in after Northampton hit the right post after gaining possession in the centre-circle from a Hunt slip which was picked up by Powell.
Two further changes were made as a result of conceding three goals. Youngster Uche coming on the Boundary Park turf for his first bit of action in a while.
Town scored their fourth of the afternoon though, through Williams once again four minutes into the second-half. Nepomuceno losing the ball on the left gave Northampton the chance to deliver a low cross in the box which was duly accepted and turned in by the number nine.
Oldham's first attempt of the second period came through Maouche. Twisting from the right to the left to create space his shot flew wide of the goal just after the hour mark.
Latics pulled a goal back minutes later through Lang after Branger's free-kick on the edge of the box was touched onto the crossbar, with the ball falling onto the line as Lang was the quickest to react.
Then Branger's long range effort on 70-minutes couldn't find the target in front of the Jimmy Frizzell stand.
The next attack that followed, Afolayan's low shot didn't have enough power on to beat ex-Oldham keeper Cornell who touched it around his post.
A succession of corners came but Iacovitti couldn't covert the last from close range despite handball claims by the defender.
The Cobblers got back involved on 78-minutes. McWilliams dragging his effort wide in the box after the ball was played back low on the ground.
But it would be Northampton who would get their fifth of the day to put the game beyond any doubt. The substitute Junior Morais with his first touch firing the ball into the net after a quick counter by the visitors.
Latics couldn't find their third after a corner from the left was headed on by Hamer and then Lang touched it towards the upright before going out of play.
Five minutes from time and Iversen made one of his trademark last ditch one-on-one saves to prevent the visitors from putting more misery onto the Boundary Park faithful.
Attendance: 5,291 (452 Away)
Oldham Athletic: Iversen, Iacovitti, O'Grady (46' Afolayan), Missilou, Branger-Engone, Hunt, Nepomuceno (65' Swaby-Neavin), Maouche, Hamer (C), Sylla (46' Uche).
Northampton Town: Cornell, Goode, Hoskins, Foley, Williams, D. Powell (78' Morais), Pierre (C), McWilliams (81' Elsnik), Turnbull, Williams, Pollock (65' J. Powell).