You have installed oh-my-zsh as a normal user, but as soon as you log in as root , you don't have oh-my-zsh available. There are probably several solutions to this problem (installing OhMyZsh separately as root, where you may end up with a conflicting environment variable when switching between your normal user account and root account), but I will now focus on a straightforward solution which gives you a copy of the whole OhMyZsh installation, without git (so without updates), which I think is the most straightforward and sufficient solution.

This is a step by step tutorial. In this example the normal (non-root) user is called normal . I also suppose you don't have to use sudo for all admin tasks (which is normally the case in Ubuntu and its derivates), but you have a separate root password. In case you don't have a root password you can simply put the sudo command in front of every command which I list here, but it would make more sense to first create a root password (which is easy, you can find a solution elsewhere).

Make sure that your root login shell is really zsh, not bash or something. Log in as root and check the name of the active shell: