Shortcut does something else

The function of a key can change depending on the screen that’s open. M is associated with Lens in the main view and Info in preview. O clears the decision in the main view and sets the scene end in preview. On the Duel screen, M means keep both: both clips stay, neither is rejected.

If you’re in a text field, leave the field first. If a window is open, the shortcut may not apply to the library behind it. Use ⌘K to check the shortcuts and the explanation for the relevant screen.