Nana Osaki (大崎 ナナ, Ōsaki Nana?) is a fictional character in the manga/anime series Nana, by Ai Yazawa. She's the titular "Nana" of the series, along with Nana Komatsu. To disambiguate between the girls, the English-language publisher of the manga, VIZ Media, uses Nana O. to refer to Nana Osaki.

Source: Wikipedia