To edit your prompt in oh-my-zsh you need to edit a PROMPT variable in your theme instead of PS1. In your .zshrc file you will find a line that looks something like this:
oh-my-zsh stores these themes in the ~/.oh-my-zsh/themes folder. If you ls ~/.oh-my-zsh/themes you will see a list of themes that you can change. The above theme would be named themename.zsh-theme in this directory.
Customizing your Theme:
If you want a simple way to customize your oh-my-zsh theme you can copy a file already in this theme folder and edit that.
To change your prompt simply edit the PROMPT variable. for example:
This would make two >'s your prompt.
I like editing the already existing simple theme. the simple.zsh-theme file looks like this: