> Can't reproduce it here.  I tried
>
>   emacs -Q --eval "(global-set-key [?\C-o] 'ff-find-other-file)"
>   C-x 5 f src/buffer.c
>
> and then in the frame that displays buffer.c I did
>
>   C-o
>
> But the problem did not manifest itself.

Did you see the more detailed recipe I provided shortly after the original
submission?