This is usually caused by the FN LOCK key being stuck. You probably also have the U, I, J and K keys behaving funny and believing they're a numeric keypad.
Try clicking two or three times both shift, the FN key, NumLock key and, just in case, the Ctrl key. Also try pressing Alt-O and Alt-M or Fn-O or Fn-Alt-O and see if that gets you back the "O" key.
If only the M and O keys behave this way, though, it looks more like a keyboard driver problem; reinstalling the driver (or selecting a different keyboard, such as French one, and then back the normal keyboard again) might fix it.
Answers & Comments
This is usually caused by the FN LOCK key being stuck. You probably also have the U, I, J and K keys behaving funny and believing they're a numeric keypad.
Try clicking two or three times both shift, the FN key, NumLock key and, just in case, the Ctrl key. Also try pressing Alt-O and Alt-M or Fn-O or Fn-Alt-O and see if that gets you back the "O" key.
If only the M and O keys behave this way, though, it looks more like a keyboard driver problem; reinstalling the driver (or selecting a different keyboard, such as French one, and then back the normal keyboard again) might fix it.