Jack Lester’s last-gasp goal, after a howler in the Oldham defence, took Chesterfield to Wembley for the final of the Johnstone’s Paint Trophy.
John Sheridan’s side will take on Barnet or Swindon Town after his warriors soaked up pressure by the bucketful from the home side.
The manager sprung a surprise by giving a start to midfielder Jamie Lowry.
The 26-year-old played for Crewe Alexandra on Saturday, the last game of his loan spell there. It was his first start for the Spireites since injuring his cruciate ligament in October 2009.
Simon Ford came in to the defence, with cup-tied duo Liam Ridehalgh and Neal Trotman missing out.
Oldham started with three men who’ve been missing of late. Zander Diamond, who was carried off in the warm up in the first leg, James Wesolowski who dislocated his shoulder just nine days ago and Robbie Simpson, hat-trick hero against the Spireites in the Latics’ 5-2 November win.
He’s had a hamstring strain and gave a boost to the home side by starting, having signed to the end of the season earlier in the day.
Diamond managed only half of the match.
Former Owl Shefki Kuqi hit the post with an angled shot just three minutes in and moments later Filipe Morais forced a reflex save from Tommy Lee with a far-post header as Oldham started well. Lee had to stoop to gather a Simpson long-ranger as the first-leg leaders struggled to build any pressure.
Scott Boden’s cross was just too far from Lowry in a quick Spireites counter-attack while Kuqi snatched at a shot to blast wide from a good position. Gregor Robertson hit the wall with a 20-yard free-kick, but Oldham continued to run the game.
Morais hammered wide from 30 yards then Drew Talbot looped one from a little nearer that Alex Cisak caught and the keeper blocked from Boden in a couple of rare Chesterfield raids.
The first half ended with a Morais shot deflecting for a corner that caused plenty of problems, then Wesolowski hit high, Oldham ending the 45 as they’d begun it, on top.
Two minutes in, Talbot had a glorious chance from Lowry’s ball but he shot at Cisak from a gilt-edged position. Lee had to save a sharp shot from Morais as Oldham began to get a grip on possession once again.
Ford’s head-blocked a box-edge free-kick from Simpson after James Hurst fouled Kuqi at the start of a spell of massive Latics pressure. Morais hit a free-kick just over after a Ford handball as the green shirted defence continued to hang on.
Oldham continued to see far more of the ball but they were unable to find the vital pass or cross. A fantastic save from Lee from a Kuqi header foiled the best of their openings.
Then, right at the end, Jean-Yves M’Voto and Cisak got in a right old tangle, allowing Lester to follow up the ball into the net to send the 937 travelling fans into heaven.
Time for a few Wembley dreams, but then all attention on to later today, the transfer window’s last day. A striker is John Sheridan’s number-one target.
Oldham: (4-4-1-1): Cisak 6, Lee 6, M’voto 7, Diamond 6 (Tarkowski H-T, 6), Black 6, Morais7, Marsh-Brown72, 6), Adeyemi 6, Wesolowski 7 (Smith 81), Taylor 7, Simpson 8, Kuqi 6. Subs (not used): Bouzanis, Parker.
Chesterfield: (4-4-2): Lee 8, Hurst 6, Thompson 7, Ford 7, Robertson 6, Talbot 6, Davis 6, Allott 6, Lowry 8, Boden 6 (Morgan 79), Lester 6 (Bowery 90+4). Subs (not used): Fleming, Whitaker, Mendy.
Goal: Lester 88.
Ref: Roger East (Wiltshire).
Bookings: Robertson, Hurst, Allott, Davis (Chesterfield); Simpson (Oldham).