The former Scotland Under-21 international was released by Crystal Palace after he failed to make an appearance during his two years.
Kettings, 23, spent three months on loan at National League side Bromley last season, playing 14 times.
He is the sixth new signing since Northern Ireland assistant Stephen Robinson was appointed as manager.
Find all the latest football transfers on our dedicated page.
League One side Oldham Athletic have signed goalkeeper Chris Kettings on a one-year deal.