The 29-year-old has previously worked with Latics manager Micky Mellon at Tranmere Rovers, where he returned to the club to the EFL.
He’s also been a Wembley winner with Forest Green Rovers when they were promoted to the Football League for the first time as they beat Tranmere in 2017.
Monthe now wants more and switched back to the north west from Northampton Town on a two-year deal.
He said: “I’m very pleased to have signed for Oldham and can’t wait for the season to get started.
“We’ve had a really strong start to pre-season, the players look sharp and there’s a great atmosphere in the squad.
“I’ve worked with the manager and a number of players before and it’s a move that really excites me.
“This is a huge club and there’s so much potential to do well, now it’s our responsibility to perform on the pitch.”