This series documents O'Brien's professional life as an actress and her interest in the performing arts, beginning with her involvement in the Irish Festival Singers to her later Broadway roles. Major productions include Da, Me and My Girl, and My Fair Lady. Materials include playbills, photographs, correspondence, newspaper clippings, and scripts for various performances.

This series consists of diaries, correspondence, memoranda, photographs, newspaper clippings, and other materials documenting O'Brien's personal life and interests. The majority of the series consists of 40 years of her diaries, which she entitled "Sylvia's Musings." In her diaries, O'Brien discusses both personal and professional matters, including her daily activities, marriage, friendships, vacations, family history, and social activities; her thoughts of various current events and social issues; and her reflections of professional matters, including auditions, particular aspects of individual productions, as well as interactions with cast members, crew, and theatrical management. The series also includes biographical and genealogical information, scrapbooks, personal photographs, and subject files on issues of interest to O'Brien.

This series consists of research notes, drafts, typescripts, and published versions of O'Brien's articles, poetry, and short stories.