The function you're attributing to the O key on your system is normally the backslash \ key which toggles between original and edited so you must have a non-English keyboard layout to the extent the O key is somewhere else or something outside of LR has remapped your key functions.
Try Tools / Adjustment Brush Overlay and change the color and whether or not the overlay is visible via the menus and see if that helps. And of course there is a Show Mask Overlay checkbox below the main image area on the Toolbar--the same bar that says Done at the right that can be used to dismiss the tool. If you don't see this toolbar, try the T key or use the View Hide/Show Toolbar menu item.
On my US-English system, the menu items described, above, indicate O and T are the the keyboard shortcuts for those menu items. If your keyboard has a different layout, maybe LR knows this and will indicate different keys? I don't have a way to test this, myself.