The script spells it with an 'O':
An explosion rocks the ship as two robots, Artoo-Detoo (R2-D2) and See-Threepio (C-3PO) struggle to make their way through the shaking, bouncing passageway. Both robots are old and battered. Artoo is a short, claw-armed tripod. His face is a mass of computer lights surrounding a radar eye. Threepio, on the other hand, is a tall, slender robot of human proportions. He has a gleaming bronze-like metallic surface of an Art Deco design.
Star Wars Episode IV: A New Hope (1977)
Although I can't find his designation used anywhere in the text of the novelization, the 1979 paperback has an inset featuring stills from the movie and short character bios; Threepio's designation appears there as "C-3PO" (with an 'O', not a '0'):
It's faded, but you can clearly see an 'O'.