The 22-year-old made 26 appearances for the Cougars last season and scored five tries.
Green, who can play in the back row, at loose forward and at prop, said he was excited by the move.
“I am buzzing to sign for the next two years,” he said.
“It was a shock, to be honest, when I found out, but really exciting. Watching what Oldham have achieved this year is something I want to be a part of.”
The Leeds-born forward came through Warrington Wolves’ academy and made five first-team appearances in 2023.
He gained Championship experience with Bradford Bulls and Widnes Vikings in 2024 before joining Keighley permanently in 2025.
His move to Oldham reunites him with head coach Alan Kilshaw.
“I'm a team-first player. Over the past season I have played big minutes and been consistent,” Green said.
“I have played all over, back row, 13 and prop. I think I’m pretty versatile so as long as I’m playing and wearing an Oldham shirt, that's all I am bothered about.”
Kilshaw said Green’s experience across Super League, the Championship and League One had helped make him a target for the club.
“At just 22 years of age, Lucas has already built an impressive amount of first-team experience, including Super League, Championship and League One football,” he said.
“The number of senior appearances he has made at such a young age tells you a lot about the potential he has.”
Kilshaw said he wanted to help Green establish himself at the higher end of the Championship.
“He’s now at an age where he needs to kick on, take the next step in his development and establish himself at top end Championship level, and I’m confident he can do that with us.”
