The Reds fell behind but young striker Victor Adeboyejo levelled seconds before half-time then substitute Moncur won it.
Barnsley head coach Daniel Stendel made ten changes from the side that beat Gillingham in the league on Saturday, potentially risking a fine for breaching competition rules.
The only player to stay in the team was Liam Lindsay who was joined captain Adam Jackson at centre-back, while Jacob Brown – making his first professional start and usually a striker – played at right-back with debutant Ben Williams at left-back.
Jared Bird and Cameron McGeehan played in central midfield with Jordan Williams and Lloyd Isgrove on the wings and Adeboyejo up front with German debutant Mike-Steven Bahre.
On the bench were first choice players Ethan Pinnock, Moncur and Mamadou Thiam as well as untested quartet Jake Greatorex, 18, Jordan Helliwell, 16, Jasper Moon, 17 and Kieran Feeney, 17.
Jack Walton played in goal in place of Adam Davies who is on international duty with Wales and had to make the first save of the game, diving low to his left to keep out Andrew Taylor’s free-kick.
Adeboyejo forced a save out of home goalkeeper Zeus De La Paz with a weak shot on the turn from 15 yards out after Lindsay headed down a Bahre corner. Walton saved again after Callum Lang scuffed a close-range effort when Barnsley’s defence was undone by a long throw.
Lang headed over a right-wing cross by Jonathan Benteke but the hosts went ahead in the 31st minute when Lindsay gave Sam Surridge far too much space to latch onto a long ball then power past the Scottish centre-back into the box before firing into the bottom right corner.
Lindsay almost made amends but shot straight at the goalkeeper, who had dropped a cross by Ben Williams who then centred for McGeehan but his looping headed was easily caught.
Walton clawed a long-range strike by Benteke – the brother of Premier League and Belgium striker Christian – out of his top left corner, then the Reds levelled minutes later.
Bird burst into the box from and was tackled but the ball rolled to Adeboyejo who, after bumping into team-mate Isgrove, poked in from eight yards out after Oldham’s defence could not react.
Ethan Pinnock replaced Lindsay at half-time, then the second half was a lot quieter. Jonathan Benteke burst into the box down the right byline and forced Walton into a save then Andrew Taylor fired over from the edge of the box.
Johan Branger-Engone beat Brown in the box but fired wide of the top left corner then McGeehan headed over a cross from substitute Mamadou Thiam.
It was another replacement who won it in Moncur with a curling 15-yard shot after Adeboyejo laid the ball off to him.
Moncur and Adeboyejo then both saw shots easily saved before Walton prevented a penalty shoot-out with a superb point blank save from Benteke who looked certain to score after heading a left-wing cross.
Barnsley now complete their group with games against Everton under 21s and Bradford City.
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