Nana O. was born March 5, 1981 in Meguro, Tokyo. She is the illegitimate daughter of Misuzu Ozaki, who left Nana in the care of her grandmother when she was four years old. Nana has not seen her mother since, and has thus forgotten what she looks like. Nana's grandmother, Miyuki, was very strict, and forbade her from wearing the colors pink and red because she did not want Nana O. to "tempt" men and become like her mother. Nana O., however, had a rebellious streak. In high school, she was friendless until she met Nobuo Terashima, who introduced her to punk music and bands such as The Sex Pistols.
She was falsely accused of prostitution and was expelled when she didn't deny the charges. Nana O. soon regretted her decision; she believes that the accusations led to her grandmother's fatal illness. However, as a present to herself, Nana bought her first red dress.
Nana O. wore the red dress to a punk concert with Nobu, where she first laid eyes on Ren Honjo. Nana was instantly captivated by him, and accepted Ren's offer when he asked her to join the band he was starting with Nobu and their friend Yasu.
As the lead singer of the band Black Stones (Blast, for short), Nana gained a rabid following of fans in her hometown. She developed a typically "cool" appearance, wearing the punk fashions of Vivienne Westwood and getting a lotus tattoo on her arm. She also moved in with Ren, and the two shared an intense romance until Ren told her that he had been offered a big record deal and would be leaving Blast to play with the band Trapnest in Tokyo. Though Ren and Nana still loved each other, they believed that a long distance relationship would be pointless. Nana proudly refused to abandon her own musical career to follow Ren; thus, their relationship ended.
Determined to "polish the shards of her dreams" and become a successful musician, Nana moved to Tokyo and re-formed Blast with Nobu, Yasu, and a new bassist, Shin. She also befriended Nana Komatsu, a kind-hearted but scatter-brained girl who, like Nana Osaki, wished to fulfill her dreams in Tokyo. The two Nanas shared an apartment (numbered 707, "Nana" being the Japanese word for seven) and became quite close despite their different personalities and aspirations. Nana gave Nana Komatsu the nickname "Hachi," which was adopted by their mutual friends. Nana also became fiercely protective of Hachi, going so far as to physically threaten Hachi's philandering boyfriend, Shoji. However, Nana is also somewhat possessive of Hachi, and feels jealous when anyone appears to be taking "her" Hachi away.
Hachi helped reunite Nana with her first love by inviting her to attend a Trapnest concert. At first Nana turned her down, but eventually gave in and agreed to go. For the first time in two years, Nana saw Ren in person. Ren saw her too, and they rekindled their romance that night. Nana thanked Hachi by introducing her to Trapnest's guitarist, Takumi, whom Hachi idolized. However, Takumi's cold "playboy" personality put doubts in Nana's mind, and she wondered if he was a suitable partner for her friend.
It soon became apparent, however, that during the period they were separated, Ren and Nana grew apart despite the fact that they still love each other. When they use to date, Ren and Nana often shared secrets and talk with one another; however, Nana now no longer confides in Ren as much. This fact worries her because whenever she has a problem, the person she turns to is Yasu, who, in turn, often rushes to her aid regardless of the time or place. Her trust in Yasu is perhaps because throughout all the years, Yasu remained an unending source of strength and a pillar of support for her, while Ren (like the mother who abandoned her) left her behind to pursue his own dreams. It was hinted that perhaps she has never forgiven Ren for it.
As the story progresses, Blast gets a record deal at Gaia Records and is quickly thrust into the public spotlight. Nana has trouble coping with the paparazzi, especially when tabloids print stories about Ren's possible affairs and probe into the location of her long-absent mother. Though Ren proposes to Nana and asks her to move in with him, their relationship is often volatile and Nana wonders if it will last. Nana also feels Hachi slipping away, since she has moved in with her now-husband, Takumi, and rarely sees her. Nana hopes to reconnect with Hachi, and their time in apartment 707 remains special in her heart.
Subsequently, both Nanas manage to catch up, first during Reira's birthday party and later, after Hachi's official engagement to Takumi, which Nana compared to as becoming "the wife of an underworld leader".
Later in the series, it is revealed that Nana's mother is actually alive, and her whereabouts are exposed by a magazine called Search, which had previously exposed Ren and Nana's relationship to the public. It turns out that Nana's mother had actually maintained a fairly normal lifestyle and even had a child of her own, Misato Uehara. The Misato that the band knew was really Mai Tsuzuki, who borrowed Misato's name to feel like Nana's real little sister. Mai was the granddaughter of a man who was a lover of Nana's grandmother, and subsequently reading her grandfather's diary became a fan of Blast and Nana. The real Misato has an incredible likeness to Nana, and when she finds out that her mother abandoned her idol, she flees in shock and disappointment.
It is hinted at that Nana disappears and is never seen again, throughout many manga chapters. She is seen, though, with long blonde hair, while there are many rumours and speculations that she is dead. A mysterious person sends Hachiko pictures of her singing, with the Ren lotus still on her arm. And Hachiko exclaims and breaks down crying, because she always believed that her friend wasn't dead, and that she waited for the day for her and Ren to be reunited again.