The versatile 18-year-old, who can operate in midfield or defence, is an academy graduate held in high regard at Old Trafford.
He formed part of Jose Mourinho’s pre-season plans over the summer and is getting a first taste of international football this week.
O’Connor has already skippered his country’s U19 side, while filling a similar role within the youth system at United, and is being tipped for big things after seeing an initial training opportunity with the Ireland team turned into a first call-up ahead of a Nations League clash with Denmark.
O’Neill said after adding the youngster to his ranks: "He’s a very impressive young man. A quiet lad, as Nicky Butt said to Roy [Keane] about him.
“But I've been very impressed with him - as a person and as a player. It looks as if he's got a decent future ahead of himself.
“You can't get carried away with things. But he's enjoyed it. He's enjoyed the training immensely. At first I think he said he thought it was intense. That's good.
“It's a great experience for him. I think that's something that will stand him in really good stead when he gets back to Manchester United.
“I don't think he's one of these lads that will get carried away with it. It will stand him in good stead and I think he will get confidence from it, which is really important."
Ireland could be forgiven for wanting to see O’Connor secure a move away from United, possibly on a short-term loan, as regular senior football will be required in order for his obvious potential to be fulfilled.
O’Neill is, however, reluctant to offer future advice to a talented teenager who has developed just fine to this point.
The Ireland boss added: “When I was a club manager, if international managers said a player should be sent out on loan, it's not something I really enjoyed hearing.
“I'm certainly far from doing that. I think it's something that players will have to look at. They will see the situation for themselves. That's outside my remit.”