When ordering, clients can select a operating system which will be automatically installed. 
To configure the client selectable operating systems, open the module page (Addons → NOC-PS For WHMCS), navigate to Settings and click Import or Update NOC-PS Operating Systems into configurable option group.
A new configurable option group will be created. Assign this group to your server products.

Setting: No Operating System

You can add the option "None" for no automated OS installation. That makes sense when clients want to install the OS over KVM.

Add the following setting into your configuration option group:

You can adjust the value No Operating System to your needs. This text will be shown at ordering at the operating system dropdown.