The 26-year-old was out of contract at from Aberdeen having spent nine years at the club, prompting the Latics to swoop for his services.
Boss Paul Dickov told the club's official website: "Zander is a player I have been personally aware of for a long time, even when I was still playing.
"He's a player I've always liked and I've watched him a lot. He's bought in to what we are trying to achieve here."
Diamond, a former captain of Aberdeen and Scotland Under-21s, becomes the latest addition to Oldham's backline after defender Jean-Yves Mvoto arrived from Sunderland and goalkeeper Alex Cisak joined from from Accrington.
Dickov added: "We want to progress as a team and we believe Zander is the sort of person that will help us to do that - he is a big character both on and off the pitch. He has also captained his club and country which shows he is a leader as well as being a no-nonsense and proper defender."
Source: Team Talk
Source: Team Talk