How can I set oh my zsh to be my default shell?

Oh My Zsh isn’t a shell. It’s a set of ZSH shell customizations like creating a colorful and grphical prompt or autocompletion and even file selection.

If you have to run zsh to get the cusomizations, then you’re not in the ZSH shell; likely you’re in Bash. You can check your current shell with the commands

$ echo $SHELL

or

$ echo $0

What you need to do is change your default shell. You can do this in Terminal; issue the command:

$ chsh -s /bin/zsh 

You can also do this via the GUI by going to System Preferences > Users & Groups Then, while holding Control click on the user you want to edit and click on Advanced Options. In the “Login Shell” field, select the shell you want to use - /bin/zsh for for the ZSH shell.

A piece of friendly advice... Oh My Zsh is a fun, colorful, and even productive add-on for Zsh that makes lots of changes and adds lots of scripts and functions to your Zsh environment. I don’t recommend this for novices. It’s always better to gain some experience in the shell environment and see how things work before installing things like this.