Midfielder with U's until January.
Moncur has spent the entire pre-season on loan at the U's but a fresh loan deal keeps the West Ham midfielder in north Essex until January 2015, subject to ratification by the relevant authorities.
Since coming through the Hammers' Academy, the twenty year old has made a couple of appearances for them and has had two spells away from East London on loan.
The U's host Oldham Athletic at the Weston Homes Community Stadium on Saturday afternoon, with Dunne also expected to name all four of his other signings in his matchday squad.
Having completed the pre-season alongside his new teammates, Moncur is expected to be in Joe Dunne's squad for the first game of the this weekend.
"It's also a step up from U21s football because you're playing for proper league points, it'll be a great experience for me."
"I'm really pleased to be here, it's a great step up for me from my two loans in League Two and Scotland," he told the official website.
He has played for AFC Wimbledon and Partick but he is looking forward to his first experience of League One football.