Borthwick-Jackson will always be remembered for his spell in the United first team under Louis van Gaal.
He said today: I'm really pleased to be back at the club and can't wait to get going again. I enjoyed my loan spell last season and I'm pleased I've got another opportunity now to build on that and show the fans more of what I can do.
“I'll give 100% commitment again and I'm focused on getting regular game time as part of a strong defensive unit."
Borthwick-Jackson joins Oldham on a 12 month deal.