Have you tried just running the commands from uninstall script by hand? It’s really straight forward: https://github.com/robbyrussell/oh-my-zsh/blob/master/tools/uninstall.sh. For the most part it just removes OMZ and attempts to restore a back up file:
rm -rf ~/.oh-my-zsh
rm ~/.zshrc
cp ~/.zshrc.pre-oh-my-zsh ~/.zshrc
source ~/.zshrc