I think that you will find that each of the glowing letters in the credits reference different blood types, with O being the most common.
Names appear on-screen beginning with a single glowing letter--almost always an "O" or another vowel when there are no "O"s--then the rest of the name and credit spreads out from that letter. (Perhaps not coincidentally, the entire regular cast, plus both creators, have an "O" in his/her name.)