Fenton worked with O'Neill at Leicester in the late 1990s and while things didn't work out for the attacker on the pitch, he claimed O'Neill didn't help him settle at the club.
"I am not afraid to speak my mind because I have read stuff in his books where has had a pop at me so, he just wasn't for me," Fenton told Tribal Football.
"I work for people who are down to earth and give information that they expect you to follow, but he was a character who didn't give clear information and then slagged you off for not doing what was expected."
After two years with Blackburn Rovers, Fenton was sold to Leicester for £1.1m in August 1997. The young attacker was actually identified and signed by O'Neill - and immediately repaid his manager's confidence by scoring on debut in a famous 2-1 win at Liverpool. But he managed just 34 league appearances over three seasons before leaving on-loan for Walsall.
Leicester lifted the League Cup in 2000 under O'Neill, but Fenton wasn't involved in the squad for the final. This despite making four appearances and scoring one goal in the run to the final.
He added: "There are players who performed well for him but I just don't like that type of person."
O'Neill is currently out of work after leaving Nottingham Forest while today Fenton is managing Northern Premier side South Shields.