The 20-year-old was released by Watford in the summer but spent most of last season on loan at Wealdstone.
Manager Gareth Ainsworth told the club’s official website: “Luke came to us highly recommended and he’s proved himself to be a very talented player who will be an excellent addition to the squad.
“He’s still young but his time on loan at Wealdstone gave him a great introduction to senior football and I’m looking forward to working with him to unlock more of his potential.”
Ainsworth admitted O’Nien may be the last permanent arrival at Adams Park this summer, adding: “Having a small squad worked well for us last year and I think 18 is a good number which we can then supplement with loan players, like we have done with Michael Harriman.
“There are still a few positions I’d like to add to and we’re in discussions with a number of clubs regarding the possibility of bringing their players to Adams Park where we can help them with their development on and off the pitch.”