The 25-year old joins the Knights after time away from the game.
After starting playing rugby league as a youngster at Askham, the Barrow born O’Neill progressed through the Academy at Castleford Tigers before making his Super League debut in 2019.
In 2021, he was selected for the England Knights Performance squad.
Across three seasons in Super League, he made 29 appearances before being released from his contract with the Tigers in 2022, to allow him to appear on the ITV reality show, Love Island.
He returned to Castleford in 2023, before being released at the end of the season.
On joining the Knights, O'Neill said: " I'm made up to sign for York.
"They're a good club with great aspirations and high standards so I'm looking forward to getting started.
"Speaking with Clint Goodchild and Mark Applegarth, they seem like great people I can work with coming back to rugby league.
"I know a couple of the boys already, like Paul McShane. He used to look after me when I was 16 so hopefully it'll change round and I'll look after him now!"
Knights head coach, Mark Applegarth said: “I remember Jacques when I was head of youth at Wakefield and he was playing at Castleford. He was always a player I knew had potential.
“Obviously he’s been away from the game and focussed on some other stuff, but when we first sat down it was very clear he wanted to make his mark in rugby league again and he wanted a bit of direction through that. It’s something that myself and chairman, Clint Goodchild feel we could offer him.
“He’s coming in for the right reasons, to play rugby league.
“I’m really looking forward to getting Jacques out there and getting him fit and rugby ready again because when he’s match fit he’s a cracking player. I think he’ll really go well this year.”
O'Neill is the fifth new signing for 2025, joining Paul McShane, Kieran Buchanan, James Farrar and Jude Ferreria at the club.