The 56-year-old, who previously worked alongside manager Sean O'Driscoll at Bournemouth and Doncaster, joined City in January.
But O'Kelly's family is still based in the Midlands, and he has chosen to step down for personal reasons ahead of the club's return to League One next season.
He told the club's official website: "The club is in a position now where, if the right decisions are made, it can only go one way.
"They are in good hands and I couldn't speak highly enough of the people I've worked with.
"I wish the club all the best in the future and am sure they will be successful."