With the adjustment brush active, press the "H" key. This will hide/show the adjustment brush.
Thanks for the advice. But sorry key H, key O and the button on the screen, they don't give me the mask I was hoping for. By pressing key O I can chech the difference between the original picture and the adjustments. So the adjustment brush works but on a rather low level.
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.
Thank you for your advice. That you reminded me to look at ³tools" was very
And yes, the button O toggles between original and edited. And the T shows
or hiddens the toonbaar with the checkbox.
Now I can use the adjustmentbrush in the best way.
Thanks a lot.