They always held the aces ahead of their game with Chesterfield as three points would mean Swansea City, the only team able to deny them, had to pull off a big win against Blackpool.
And in the end Oldham achieved their target with plenty to spare.
Victory was provided by Chris Porter's 23rd strike of the season while Swansea were hammered 6-3 at the Liberty Stadium by Blackpool.
The Latics are in the play-offs for the first time since 2003 but this was a fitting end to Chesterfield's campaign as they were already doomed to spending next season in League Two.
Oldham made a bright start at Boundary Park and winger Andy Liddell shot just wide of the far post in the second minute.
There was another half-chance which Porter sent off-target after good work by Chris Taylor before Chesterfield striker Colin Larkin had a shot deflected over the bar.
Larkin wasted Chesterfield's only real chance on 33 minutes when he seized on a loose ball 18 yards out, but he was unable to slot it into the net with plenty to aim for as Oldham keeper Les Pogliacomi strayed off his line.
Liddell also squandered an opening before Oldham made their breakthrough in the 58th minute.
Simon Charlton sent over a free-kick from the right and Porter jumped high to power in a header from six yards.
The Latics could have doubled their lead when Paul Warne headed off target but they were never in any danger of Chesterfield denying them.
After the win, Oldham manager John Sheridan was delighted, saying: "It's a brilliant achievement by the lads and a lot more than most people expected from us at the start of the season."
Chesterfield boss Lee Richardson said: "We've had 11 games this season which have ended 1-0 and we've lost ten of them. That tells you something and we need to put it right next time."