How to install Antigen if I have installed Oh My Zsh previously?

In the official Antigen repository there is no instruction for installing this manager for people that have install Oh My Zsh before. I asked above question in the program's repo but nobody answered it. In the process of installation Oh My Zsh rewrites .zshrc configuration file completely. So what I should do to properly install Antigen if I have Zsh + Oh My Zsh installed previously? (Nothing, uninstall Oh My Zsh, restore original .zshrc file or smth)