Alright, I was able to fix this by adding hints to the event listeners, and rewriting my Listener.php file to look like this:
<?php
class OPOnly_Listener
{
public static function loadClassController($class, array &$extend)
{
$extend[] = 'OPOnly_ControllerPublic_Thread';
}...
I'm pretty new to XF development...I'm working on an extension, and my controller extensions are working fine, but all my model extensions are resulting in a Cannot declare class XFCP_<class_name>, because the name is already in use error.
There aren't classes with the same names as my model...