On Ollie, you can change your settings so that either:
You can only order by full cases
You can order by cases or by individual units
To change your settings, go to your Settings page, click on Orders.
Scroll down to the Quantity Preferences section. There, you can toggle between ONLY SELL BY CASE: ON or OFF.
If you turn it to OFF, then you can set your order default preferences to Units or to Cases.
As long as you have turned OFF only sell by cases, then on the order you can still toggle between cases or units, despite your default setting: