And do what? Take it off "Read Only"? Been doing that since the game came out. That is for all the hotkeys. If it is like that they will re-map, but you will lose them when you quit the game. The (O) key is entirely different. The slot will not accept any input at all. When you try, the (O) disappears (loses it's preset) and nothing will show up in the slot and the key(s) don't activate Engine Doubling. This condition also happens with the (\) key if you try to map it to anything, but at least the key does function when remapped, the slot just looks empty.
EDIT: If you try to map (\) to Transporters, it reads in the slot as a 0, but the (\) key does work for them. Other slots appear empty with that key, but they do function as mapped. Weird.