Oh! My Goddess or Aa! Megami-sama, as it is also known, is a fantasy romance series that pertains to the seinen genre. It has been serialized since 1989 and it hasn't ended yet as of today.
It is the story of a college student named Keiichi Morisato, a guy who loves motorcycles and cars, but is constantly bitter on his bad luck with women due to his height. One day he meets a goddess called Belldandy by accident, while he was trying to communicate on the phone to send a message.
Belldandy offers her services, granting him a wish. Morisato, finding her extremely hot, asks her to stay with him forever and his wish is granted. The fun ensues when Belldandy's sisters, Urd and Skuld, move on with them as a consequence.
There has been OVAS, a movie and the anime series, but all three of them haven't been exactly accurate to the manga. Here's one of its classic anime openings XD:
