Now I've tried to look at some of the FAQ on the O&O site.
The issue may relate to how the app was installed?
They offer two options for installation:
1 "When you download O&O Defrag from our website or from a link sent by our Online Shop, you receive a simplified installation. This installs O&O Defrag in the standard configuration and sets optimized settings for your computer during the new installation. Once installation’s completed, O&O Defrag, without any further action on your part, starts working in the background to improve your computer’s performance."
2 "In the event you don’t want the standard installation, click Advanced installation in the reduced installation dialog. Once you do, the classic O&O Defrag installation wizard with all the usual configuration options will then be opened."
My guess is that the option for classic mode only is available using the advanced installation option.
[Login to see the link]