O'Donovan, who played under Roy Keane at Sunderland in the English Premier League, makes the move to the A-League saying he is 'hungry to do well'.
"I'm very excited and looking forward to playing in the A-League and representing the Central Coast Mariners, I was up there last week and it's a beautiful place and looks like a terrific place to play football," O'Donovan said.
"It's [the A-League] a well-respected league and it's a league that I've always felt that I wanted to be a part of and I'm now very proud to say that I am a Central Coast Mariners player.
"Playing with Sunderland has been one of the highlights of my career. Roy Keane was an idol of mine growing up and it was terrific to play under him and to play at that level of football was an incredible learning curve for me.
"I'm hungry to do well at the Central Coast Mariners. I consider myself a number nine, I like to play off the shoulder of the last defender and being a nuisance to central defenders and hopefully a goalscorer."
Starting his career with Cork City, 29-year-old O'Donovan made the move to Sunderland before embarking on loan spells with Dundee United, Blackpool, Southend United and Hartlepool United.
He then made a permanent switch to Coventry City, and finished his career in the UK with a loan spell with Hibernian and a move to Northampton Town.
From there he enjoyed a stint in Singapore's S-League with Brunei–based DPMM FC, before an ill-fated switch to Indonesia and Mitra Kukar.
"Roy comes with outstanding references from top managers in the UK including Roy Keane," coach Tony Walmsley said. "He is a very humble and genuine character who will fit our community club profile.
"To have a striker join the Mariners with an identified work ethic who is also a proven goalscorer with natural pace and speed of thought is very exciting. The type of striker Roy is, will give our attacking midfielders room to operate with his good forward runs.
"With our ability to get service in from both flanks and our creativity in midfield I'm expecting a high return from Roy this season."