FitzGerald quit last week after just 13 months in the job with theories that O'Neill's total hands-on role with regards to player contracts may have been a factor.
But O'Neill insists they enjoyed a positive working relationship.
"In fact, it was quite the opposite. I got on very well with him. I really do not know the reason he left.
"I think his job from his own particular background (in sports marketing) was based on other issues. I'm running this side of events at Bodymoor Heath. I said to the owner I will take complete control of the training ground and everything to do with the playing aspect.
"Now sometimes there's a crossover of responsibilities, but I'd be hoping the brand or commercial side of Aston Villa Football Club is similarly upheld. All you can do is uphold your own end. If the team are going well, I think it's always easier to sell certain commercial activities, such as boxes, and if we're not doing so well, it rebounds.
"But that side of events is something I couldn't do. I will help if I'm asked to help promote something I genuinely believe in and we think is worthwhile.
"Other things I'd want to stay clear of because I'm totally unaware of. I know it's about people putting their hands in their pockets and coming to games but I'm here to run the football club."