Go to your postbit/postbit legacy template.
Look for where you want to add the (OP) text for the thread starter, and paste this code there:
Code:
<if condition="$thread['postuserid'] == $post['userid']"><div class="smallfont"><b>(OP)</b></div></if>
Note: You can change this code around, remove the boldness, etc. You can also make it say something other than (OP), maybe Thread Starter?
Anyway, enjoy!