The initials O.T. can stand for a great many things (overtime, occupational therapy, observation tower, the Operating Thetan in Scientology, the Nazi Organisation Todt, the Old Testament, and so on). But if you're standing in a Spanish train station, it might also help to know what they're supposed to mean on the sign pictured to the right. Ot the is a typo of extremely "high probability" on the Ballard list since it occurs a whopping 1,116 times in OhioLINK. Most of the time, it appears to be a typo for of the or to the, but in some cases it could be standing in for the prepositional phrases or the or on the as well. No matter which way you go with this one, you'll be doing your catalog a big favor by fixing this tiny typo.
(Directional poster in a Spanish railway station, from Wikimedia Commons.)