Add this to your `.zshrc` file, _after_ sourcing Oh-My-Zsh:
```zsh
bindkey '\t' menu-complete
```
Now <kbd>Tab</kbd> will behave exactly the same as <kbd>Shift</kbd><kbd>Tab</kbd>, but in the opposite direction.

---

Alternatively, for more control over how completion behaves, install my [Zsh Autocomplete plugin](https://github.com/marlonrichert/zsh-autocomplete).