this is a staff only feature however it may be possible to ask someone to do it for you

http://forum.toribash.com/chat_view.php?channel=support

EDIT: turns out:

[13:44] <sir> I'd actually rephrase it, OP being visible on every page is a board setting
[13:45] == SmallAway has changed nick to SmallBowl
[13:45] <sir> If it's enabled, any thread has it
[13:45] <sir> If it's disabled, even admin threads can't have OP displayed on every page