Key for the letter "o" stopped working, works only with Shift, Linux

I'm going to guess you're using the Unity desktop, since that's the default for Ubuntu and uses Compiz.

Click the gear icon, top right, and "System Settings". Then select "Keyboard" from the hardware section.

In the keyboard settings window, you want the "Shortcuts" tab (not the default one with speed sliders).

Search the categories for a key mapping that is assigned to "o".

If you don't find one there then it could be Compiz itself, but you'd know if you'd been messing with the settings in there, so it seems unlikely. Anyway, the configuration tool is called "CompizConfig Settings Manager", and is not installed by default.

If all else fails, delete (or move) your .compiz directory, and/or your .config/compiz settings. It might also be in the equivalent unity or gnome settings in .config or .local/share, but exercise extreme caution in those folders or else you'll break other applications.

Open a terminal and run xev. Then press o on the window that appears and see what xev shows. If it's different that what you get for other keys then your keymap is most probably severed somehow.

Try running xmodmap -pk. This should show o and O at the same line.

Try switching to a console (alt-ctrl-f1) and see if o works.

Fire up a new X server with just a terminal and see if it works there:

X :1 & sleep 3 ; DISPLAY=:1 xterm & 

Open a terminal and run (copy-paste) this, after ensuring that you have xdotool installed:

sleep 1; xdotool type o ; echo

The above should narrow down your search.

I had exactly the same problem as the one described (except that I use Ubuntu 16.04): "o" did not work but "O" did work (I noticed later that the same happened with "z" and "Z"). I went through some forums and nothing seemed to help other than reinstalling Ubuntu... there had to be something else.

Then, I remembered that I had installed some time ago the "Unity Tweak Tool" which allows, among other things, to zoom in and zoom out your Unity desktop. Here the "Desktop magnification" option was enabled, the "Zoom in" had the shortcut "z"... and the "Zoom out" had the shortcut "o". Since "o" is way more common than "z", I only noticed that "o" did not work (in fact it was working: it was zooming out a desktop that was not zoomed in i.e. it did nothing). So I replaced the shortcuts to Ctrl+Alt+z and Ctrl+Alt+o and everything works as expected now, all letters work normally and I can zoom in and zoom out at will.

Even if your problem is not the same, the letters that are not working are probably used as a shortcut somewhere.