Forest 2-3 Oldham
Three goals in seven second half minutes turned the tide of the third round tie in the favour of the League One strugglers.
Forest had taken an early lead, courtesy of a Matt Smith own goal, and looked comfortable for much of the opening 45 minutes.
But two goals from Robbie Simpson and a Jose Baxter strike sent Oldham into the draw for the fourth round of the competition.
To make matters worse for The Reds they had to play for the final 30 minutes with just 10 men after captain Danny Collins was red-carded for a challenge on Smith.
Collins was adjudged by referee Roger East to be the ‘last man’ as he brought down Smith as he bore down on goal and will miss next Saturday’s home date with Peterborough through suspension.
Substitute Billy Sharp bagged a late consolation goal but the day belonged to Paul Dickov’s Oldham.
Boss Alex McLeish made five changes to the Forest side beaten at Blackburn on New Year’s Day with Simon Gillett, Chris Cohen, Brendan Moloney, Dexter Blackstock and Radi Majewski returning to the starting line-up.
Guy Moussi, Henri Lansbury and Sharp were relegated to the bench, while Daniel Ayala and Adlene Guedioura were unavailable.
An injury sustained in training kept Ayala out of McLeish’s plans, while Guedioura could be absent from The City Ground for up to a month depending on Algeria’s progress in the African Cup of Nations.
There was little hint of the drama to come when The Reds took the lead after just 13 minutes of a first half they largely controlled.
Reid sent over a free-kick from close to the corner flag on the right hand side and although Cox claimed the goal, Oldham defender Smith appeared to inadvertently get the decisive last touch.
Majewski went close to making it 2-0 midway through the first half with a shot from distance after Gillett had cut-out a sloppy pass from Lee Croft in the centre-circle.
Gillett was at the heart of Forest’s next bright moment. His perfectly-weighted ball forward found Cox racing into the area and although his progress was blocked, the ball broke kindly for Blackstock but, just as he was about to shoot, Jean-Yves M’Voto managed to clear the danger.
A mistake by M’Voto’s central defensive partner Cliff Byrne presented Blackstock with a clear run at goal but ‘keeper Dean Bouzanis raced off his line to block his progress.
Baxter provided the odd moment of hope for Oldham and he went close with a free-kick from 20 yards out 12 minutes before half-time.
Forest were forced to make a change shortly afterwards as Reid, who had picked up a knock in a tackle, was replaced by Lewis McGugan.
Simpson cleared a Blackstock header from a corner off the line as Forest looked to add to their lead before the break.
Oldham livewire Baxter again tried his luck from distance in the dying seconds of the half but his shot curled harmlessly wide of Lee Camp’s goal.
Oldham, who now attacking their 2,841 supporters in the Bridgford End, made a bright start to the second half and were rewarded with an equaliser just nine minutes in as Simpson beat Camp with a shot from 25 yards out.
The visiting fans had even more to celebrate just four minutes later as Simpson bagged his second, sending a looping header over Camp from Dean Furman’s right-wing cross.
Simpson was denied a five-minute hat-trick when Camp blocked his shot with his legs but things went from bad to worse for Forest just seconds later.
Collins was shown a straight red card after bringing down Smith on the edge of his area and Baxter made it 3-1 from the resultant free-kick which appeared to take a deflection off the wall.
Forest had appeals for a penalty waved away when Byrne sent Blackstock tumbling to the ground just moments before McLeish introduced Sharp to the action at the expense of Majewski.
Cohen almost gave handed the home side a lifeline with a cross-cum-shot from a tight angle that Bouzanis managed to claw out.
The Oldham ‘keeper didn’t exactly cover himself in glory just moments later, however, as he failed to hold a long throw from Halford into his six-yard box but Forest were unable to capitalise on his mistake.
Camp made a fine save to deny Oldham a fourth, palming Youssouf M’Changama's curling long-range effort behind for a corner.
At the other end McGugan had a shot deflected just narrowly wide of the target as Forest looked for a foothold in the contest.
Sharp pulled a goal back in the fourth minute of added time as he hooked a shot over Bouzanis but it was too little too late.
Forest: Camp, Cohen, Halford, Collins(c), Moloney, Harding, Gillett, Majewski (Sharp 66'), Reid (McGugan 37'), Cox, Blackstock
Subs not used: Evtimov, Lascelles, Moussi, Lansbury, Greening
Scorers: Smith (og) 13', Sharp 90'
Red Card: Collins 59'
Oldham: Bouzanis, Grounds, M'Voto, Croft (Millar 90'), Furman(c), Simpson (Taylor 82'), Smith, Byrne, Baxter, M'Changama, Wabara
Subs not used: Cisak, Brown, Tarkowski, Winchester, Mellor
Scorers: Simpson 54', 58', Baxter 61'
Booking: Grounds 37'
Referee: Roger East
Attendance: 11,293 (Away: 2,841)