Sometimes OO-Browser may end up in a state, where there are variables and structures incorrectly initialized in the memory. Then it will cease to function correctly, displaying seemingly unrelated error messages. This can happen if you interrupt an operation, jump to a buffer not expected by OO-Browser, and for other reasons. Sometimes the only remedy is to exit Emacs and start again. Sometimes a combination of C-c C-r, C-c C-l keystrokes will recover from the situation.
OO-Browser seems to work best with XEmacs, especially the mouse support is better. I have tried it with FSF Emacs as well, with mixed results. Under Emacs 19.29 on Alpha-OSF 3.2, I managed to get FSF Emacs to dump core. Under Linux however, Emacs 19.29 seems to work fine.