If you ever find that you've done something silly like

 bind -n o select-pane -R
 bind -n O select-pane -L

Then reloaded your tmux config live. Now when ever you type o or O you're switching panes left and right. No problem easy fix just remove the -n

 bind o select-pane -R
 bind O select-pane -L

Now it works as intended, but wait. Tmux does not automatically unbind keys. So how do you get your Os back?

Simply detach from the session and tell tmux to unbind them

 C-a d # note I have my prefix bound to C-a
 tmux unbind o
 tmux unbind O
 tmux attach

Prest-O!