Born the last of five daughters to the Commander of the Royal Guards, General François Augustin Regnier de Jarjayes (a real historical personage), she is raised by her father as if she were a boy in order to take his place and command the Royal Guards serving at Court after he retires. At the age of fourteen, as soon as her training in the basic military skills is complete, Oscar is given the task to protect the Dauphine Marie Antoinette when she arrives at the French Court.
Despite being raised as if she were a boy and dressing in males' clothes, Oscar is open about being female. Even as she embraces her femininity, she uses her male position to gain freedoms that she could never have as a lady of the court.
She is the love interest of both André Grandier, her servant at the Jarjayes mansion and afterwards a soldier in her regiment, and Marie Antoinette, who she acts as a bodyguard to. She also earns the admiration and love of Rosalie Lamorlière, and in turn calls Rosalie her "spring breeze". Other women are infatuated with Oscar, even after she tells them she is female. She dislikes the court intrigues, but remains there out of loyalty to her father and her friend, Marie Antoinette. At one point, Oscar falls in love with Fersen, who has a forbidden love for Marie Antoinette.
Soon gaining the Dauphine and Queen-to-be's friendship and trust, Oscar experiences life at Versailles and the pain caused by the contradictions of her being a woman whom everybody, including herself, considers a man. Later in the story, Oscar learns of the political ideals of the Revolution and that the royalist regime is corrupt. As the French Revolution is about to begin, Oscar refuses to sedate tumults occurring in Paris. She renounces her status and her regiment joins forces with the people marching to the Bastille. Shot by soldiers inside the fortress, she dies right before the prison falls, but not before seeing a white bird flying over her, indicating that bird is Andre, who died the night before her death, waiting for Oscar to come with him to Heaven. They are buried together on a hill.
Oscar was originally a supporting character to Marie Antoinette, and was created as a female because Ikeda was unsure if she could accurately portray a male soldier. Oscar eclipsed Marie Antoinette in popularity and due to reader feedback became the main character. Eri Izawa suggests that as Oscar is fictional, Ikeda could be freer in the portrayal of Oscar's life than Ikeda could be in the life of Marie-Antoinette, who had to die on the guillotine. Oscar's androgynous persona is based on the actresses who play male roles in the Takarazuka Revue and Princess Sapphire, and she was named for Oscar Wilde, as Ikeda is a fan of his work.