Jack Midson scored a second-half equaliser for the visitors after Stephen Gleeson had hit a thunderous strike on the stroke of half-time but the right-back struck at the death
At the end of a muted first half, Stephen Gleeson gave the home team the lead with a brilliant strike from 30 yards.
AFC Wimbledon's attacking threat was limited but Jack Midson took full advantage of the only sniff of goal presented to him by heading in Toby Ajala's excellent cross.
And after the visitors' captain Steven Gregory had failed to take a glorious chance to seal a fairytale win at the death, Otsemobor calmly flicked home in a penalty area melee.
The atmosphere prior to kick-off at Stadium MK was raucous; the AFC Wimbledon fans booed the every mention of their opponents by the stadium announcer while the home supporters cheered in response. For all the pre-match sideshow this encounter has supplied, though, there remained a football match to be played and after the referee blew his whistle emotions were more tempered.
MK Dons, as you would expect of a team sitting in third place in League One, began the match in the ascendancy but manager Karl Robinson is known for his insistence that his side pass the ball out of defence and it took them some time to find their rhythm. Goalkeeper David Martin needed to be alert to sweep up more than one nervous pass back into his area.
They eventually settled, but lacked a creative spark in the final third despite the best efforts of former Manchester United midfielder Luke Chadwick. Angelo Balanta, who is on-loan from QPR, received a pass into feet on the edge of the box and turned his marker but shot wide of Neil Sullivan's left-hand post.
AFC Wimbledon rarely ventured out of their own half and on the stroke of half-time, the hosts finally found the moment of quality they needed to take the lead. Shaun Williams stepped into midfield and rolled a pass across the pitch to Gleeson, who rocketed a superb 30-yard shot beyond the reach of Sullivan and into the top corner.
That seemed to lift a weight off the shoulders of MK Dons as they attacked towards the home fans in the second half. Balanta saw one effort from the left-side of the box deflected just wide of the post after Dean Bowditch capitalised on a slip by Steven Gregory, and most of the stadium thought Bowditch had doubled the lead when he volleyed into the side netting.
Then, with their first real attack of the game, AFC Wimbledon equalised. The warning signs were there; MK Dons had taken their foot off the pedal and allowed Neal Ardley's team more control of the ball in midfield. Ajala was set free down the right flank and Midson headed past Martin to provoke a minor explosion in the away end. 20 or 30 fans spilled over the hoardings and onto the pitch, but there were no serious confrontations before they were returned to the stand.
MK Dons desperately struggled to create chances thereafter and when Gregory faced Martin at the death, the home fans feared the worst.
But the midfielder's shot was weak and turned around the post by Martin and after Otsemobor forced a corner with a brilliant run, the right-back flicked home substitute Zele Ismail's shot in the subsequent mayhem to win it.