Gardner, 27, is the Latics' first signing this summer and joins after he turned down a new deal.
After a short stint in the Celtic youth system he has also played for Crewe and Halifax as well as loan spells at Bury and Tranmere.
Edmundson, 19, has signed a new two-year contract with the club.
He came through the Latics youth system and made his debut in September 2015 at Shrewsbury, with the deal including an option of a further year.
Find all the latest football transfers on our dedicated page.
Oldham have signed midfielder Dan Gardner on a two-year deal from Chesterfield and agreed a new contract with defender George Edmundson.