One problem with that last one is that Outlook Express (and possibly some other software which pretends to be a newsreader, but I've only verified it on OE) take *any* two-letter pair followed by a colon ':' as being "Re:" in some language or other, strips it off, and replaces it with its own "Re:". I've seen it happen *many* times in this newsgroup.
So -- something other than "OT:" is needed. Perhaps "[OT]" will work well enough, and not be wiped off branches of the thread map by anyone replying with OE as a newsreader.
Enjoy, DoN.