The File Open dialogs are not replaced by Opus.
Explore Replacement covers the Windows Explorer / File Explorer application, e.g. when opening folders from other programs. It doesn't cover the File Open dialogs (for choosing files) or the rest of the Windows Shell (taskbar, desktop background, etc.).
Windows doesn't provide a good way to replace the File Open dialogs and anyone who has tried to gave up after a while as it is not possible to avoid breaking things that modify the dialogs.
There are some tools which enhance the standard dialogs, rather than trying to replace them, and several of those will talk to Directory Opus to give you quick access to folders which are open in Opus windows/tabs, or (if my memory serves) the Opus favourites list. Those and other details were discussed recently here: