ΘΣ (Theos) seems needlessly messianic but it was there in plain sight in The Pandorica Opens, of course. ΟΣ = Hos = "who" sounds like a character off Bonanza.
No, it's Theta Sigma, the Doctor's nickname at the Time Lord Academy as established in the 1979 serial "The Armageddon Factor." That's what the writing in "Pandorica" was alluding to. Whether Bob Baker & Dave Martin intended an allusion to the classical abbreviation of "Theos" is unknown.
I should have added -- "needlessly messianic" is part of a quotation from HHGTTG, of course. I have no proof, but it seems to me that the choice of ΘΣ was deliberate by Martin and Baker who came up with Omega, which also has a religious connotation. UK universities and colleges don't have fraternities or sororities so I doubt that any association with their naming was intended.
Ooh, that's a good one. Indeed, it was the Doctor (or technically Ian) who showed cavemen how to make fire all the way back in the first serial, so he really is Prometheus, and a "lightbringer." (Not to mention that the folk etymology for "Prometheus" is that it means "foresight" or "forethinker" -- which could be taken to mean "one who sees the future." Though it's now believed to mean "thief," and the Doctor is certainly that.)
We already have a great slobberer...er...I mean Rassilon, in Timothy Dalton. Cumberbatch could be an excellent Master though, if they're ready to move on from the goofy John Simm version.
Actually, I posted that on the basis of Cumberbatch's Star Trek performance, which reminded me somewhat of Dalton as Rassilon. Back on-topic: Whee the fibble.