Roy Keane has left his role as assistant manager of Nottingham Forest to restart his own managerial career.
Having worked under O'Neill for five years with the Republic of Ireland, Keane followed him to Forest in January, but Sky Sports News understands the former Manchester United captain is now seeking a return to a top job.
After beginning his managerial career with Sunderland, Keane spent two years at Ipswich, who he left in 2011, before linking up with O'Neill in his Ireland role in November 2013.
"Working with Martin over the last few years has been a magnificent experience, one of my greatest in football both as a player and a coach and one I want to personally thank him for," Keane said in a statement released by Forest on Sunday.
It is understood there is no animosity between O'Neill and Keane, who explained the timing of his departure is intended to avoid mid-season disruption during Forest's 2019/20 Sky Bet Championship campaign.
It is thought Keane does not have a specific job opportunity at the moment, but is open to offers.
A Forest statement read: "Everyone at the club would like to wish Roy the very best in the next stage of his career."