The 19-year-old, who spent time on loan with Tranmere last term, has agreed terms on a two-year contract with Latics following his release from Premier League side Everton.
Green revealed ex-Oldham boss Joe Royle was instrumental in the deal, telling the club’s official website: “Joe Royle at Everton pushed me towards Oldham because of his relationship with the club and I managed to get the move and I’m delighted to be here.
“The gaffer has got a good footballing philosophy and he conducts himself very well, so I can’t wait to work under him.”
Kelly has added Green to his earlier acquisition of returning winger Lee Croft.
“George is an excellent creative player who can open things up and will be a massive asset to the team,” Kelly said.
“He is excellent on the ball and his delivery into the box is absolutely superb and I’m excited to work with him.
“After speaking to George, he is hungry and cannot wait to get cracking.
“He is still a young lad and his ambition matches mine in terms of being successful, so I’m delighted to have him on board.”