How do I turn off the built in macOSD shortcut for Minimise ('Opt' + 'Cmd' + “M”)

You can change the designated keyboard combination. Option-Command-M is actually Minimize All

Go to System Preferences> Keyboard> Short Cuts in the left hand Column click on App Shortcuts.

Now click on the + button at the bottom of the right hand panel and select All Applications.

At Menu Title type in Minimize All and in the Keyboard Shortcut box enter the keyboard combination that you would prefer to use try and make it unique. Click Add the new command should now be available to all apps that have that command. You can do the same for the Minimise (Command-M) key combination also if you want.

Hold down the Option key while looking at the menu. The Option key gives you…Options.