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.

The decommissioned Transocean Winner came ashore at Dalmore, near Carloway on Lewis, in a storm while being towed to the Mediterranean.
Its owner Transocean has paid the money requested by the MCA to cover costs in dealing with the incident.
The rig was eventually taken back under tow and scrapped in Turkey.
The rig's grounding last August sparked pollution fears due to the 280 tonnes of diesel on board.
Investigations found two of its four fuel tanks were damaged in the incident which resulted in the loss of 53,000 litres of fuel, most of which is thought to have evaporated with no damage to the environment.
Three weeks later it was towed to the safer anchorage at Broad Bay before being floated on to the deck of the Hawk, a semi-submersible ship used for transporting large maritime structures over long distances.

Now, none of her albums are available on the music-streaming service, after the singer removed her entire back catalogue.
So far there's been no word from Swift on why she's taken the tracks down.
Spotify meanwhile has released a statement saying that the company hopes "she'll change her mind" and return to the site.
If you're really desperate for some Taylor Swift - Newsbeat has managed to find two tracks on Spotify that seem to have been left behind.
Love Story still features on a compilation called Heart Breakers and The Hunger Games soundtrack has Safe & Sound on it - but there's no guarantee they'll be on there for long.
"We love Taylor Swift, and our more than 40 million users love her even more - nearly 16 million of them have played her songs in the last 30 days, and she's on over 19 million playlists," said Spotify.
"We hope she'll change her mind and join us in building a new music economy that works for everyone."
They added, in reference to one of her hits, Stay Stay Stay: "PS - Taylor, we were both young when we first saw you, but now there's more than 40 million of us who want you to stay, stay, stay. It's a love story, baby, just say, Yes."
Representatives for Swift have yet to return a request for comment.
The 24-year-old's fifth album, 1989, was released last week and has topped the album charts in the UK.
It sold 90,000 copies, making it the fastest-selling album of the year by a female artist, but it was not put on Spotify.
Swift has been outspoken in the past about music piracy and in an article in the Wall Street Journal earlier this year, she described how the business models used by some artists have devalued music.
"Piracy, file-sharing and streaming have shrunk the numbers of paid album sales drastically, and every artist has handled this blow differently," she said.
"In recent years, you've probably read the articles about major recording artists who have decided to practically give their music away, for this promotion or that exclusive deal.
"My hope for the future, not just in the music industry, but in every young girl I meetâ€¦ is that they all realize their worth and ask for it."
Spotify claims that its business model, in which revenue is shared with the "music community", helps to tackle the issue of piracy.
"We believe fans should be able to listen to music wherever and whenever they want, and that artists have an absolute right to be paid for their work and protected from piracy," it said.
"That's why we pay nearly 70% of our revenue back to the music community."
The site has compiled two playlists in response to her departure, one of which is called What to Play While Taylor's Away.
The other - A Little Playlist Poetry for Taylor Swift - features tracks whose titles when read in order say: "Hey Taylor We Wanted to Play Your Amazing Love Songs And They're Not Here Right Now."
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League One side Oldham Athletic have signed goalkeeper Chris Kettings on a one-year deal.
[NEXT_CONCEPT]
The owner of a 17,000-tonne oil rig which ran aground on the Western Isles last year has paid the Maritime and Coastguard Agency (MCA) £400,000.
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For the last few weeks, Taylor Swift's Shake it Off has been one of the most streamed tracks on Spotify in the UK.