Highly-rated O'Brien, 19, last week penned a one-month loan deal at the New Bucks Head from League Two outfit Salford City.
He has been drafted in by boss Wilkin while injuries to Matty Brown and Jordan Piggott leaves the Bucks short at the back. O'Brien's likely debut in the FA Trophy at Solihull Moors on Saturday was postponed and pushed back until Tuesday.
The centre-half, Dublin-born and raised in the United States from the age of 10 weeks to an Irish father and German mother, has earned positive reviews from Neil Wood, manager of the Ammies, after two starts in League Two this term, both in October, having checked in from Watford after one season in the Hornets' academy.
"Yes I think he's training regularly with the first team there," Wilkin said of his latest loan addition.
"He's made first-team appearances, I think he's made a couple of league appearances within that and not just the Papa John's Trophy, which a lot of EFL clubs will give their (young) players experience in that.
"And a lot of agents and certain clubs will promote them off the back of the Papa John's.
"I think the real acid test comes when you can force your way and get a league start against your name, that's the biggest tick and recommendation you can probably get.
"Josh has at least a couple of starts and I know speaking to Chris (Casper, sporting director) at Salford that he's very highly thought of.
"He's 19, young men making their way and learning their way is what you have to understand and accept.
"It can come a little bit soon for some of them, there are certain players that are able to cope with this level at this time in their careers and I'm very optimistic that Josh will be one of those that goes and grasps it and does well."
O'Brien's first senior league start came at Northampton Town earlier this season, where he helped Salford keep a clean sheet – despite only learning of his surprise start when he arrived at Sixfields.