After uninstalling Oh-My-Zsh, my terminal is messed up
Because you are checking the "default login shell" option, Terminal will start your default login shell, which is apparently zsh. What you enter in the box below makes no difference, because you do not check that option.

You do not state what your goal is, but I will assume that it is to return to bash, which is the default in macOS.

To change your login shell back to bash:


Open Settings (from the Apple menu).
Select "Users and groups"
Unlock it, if necessary, by clicking the padlock icon at the lower left and entering your password.
Control-click your username in the list at the left, and select "Advanced" in the menu that appears.
In the dialogue sheet that appears, change your login shell to /bin/bash.