Aa! Megami-Sama! is the original Japanese title, as mentioned above, but I've seen both Ah! My Goddess and Oh! My Goddess. To english-speakers, Oh! My Goddess will likely sound more familiar, but I am very much used to Ah! My Goddess as the title (mainly because it is the search term that yields episode streaming and it sounds sightly more elegant). However, one is free to call it by the title one chooses
