The striker has opted not to renew his contract with Tommy Dunne's side after suffering a difficult 2012 season through injury
The 29-year-old forward joined the Leesiders in 2009 from Cobh Ramblers and was a key member of Tommy Dunne's side. Speaking to the club's official website, Dunne said that O'Neill had suffered a difficult 2012 season with injuries and thanked him for his services to the club.
"I spoke to Davin and he won't be signing back with us next season. He was unlucky last season in that he had quite a few injuries," he told corkcityfc.net.
He added: "He is a fantastic professional and has been a great guy to have around the dressing room, as well as being an excellent player for us on the pitch.
"I would like to thank him for everything he has done for the club and wish him the very best of luck for the future."
The Cork boss also revealed that negotiations were still ongoing with a number of players ahead of the forthcoming season and stressed that he would not rule anything out.
"There are a couple of players who were at the club last season who have not yet signed back for next season, and we are still talking to some of them," he said.
Meanwhile, Derry City boss Declan Devine is hopeful that striker David McDaid will remain at the club for the 2013 season.
The 22-year-old forward has been offered an improved two-year deal with the Brandywell club but has been linked with a move to Sligo Rovers after his proposed transfer to Fleetwood Town broke down.
"We've offered Davy a two year deal as I'm very keen to keep him at the Brandywell now that the Fleetwood Town move does not appear to be working out for him," Devine told the Derry Journal.
"He had an excellent season with us last year. He’s a local player who ticks all the boxes and a player I hold in high regard," he added.