Born in Cotu Vameș, Neamț County, she was the fifth child of schoolteachers Gheorghe Gavrilescu and his wife Ecaterina (née Petrovici). She attended middle and high school in Iași and took courses at Iași University's literature and philosophy faculty, but did not graduate. Her pen name, which she never liked, was selected by her mentors Mihail Sadoveanu and Garabet Ibrăileanu: the former came up with "Otilia", the latter with "Cazimir". In 1912, she made her debut with poems in Viața Românească, to which she remained a loyal contributor. Other magazines that published her work include Însemnări ieșene, Adevărul literar și artistic, Lumea, Bilete de Papagal, Iașul nou, Iașul literar, Orizont, Gazeta literară and Cronica. Her first book was the 1923 poetry volume Lumini și umbre, followed by Fluturi de noapte (1926) and Cântec de comoară (1931)
Cazimir's poems focus on the universe becoming domestic, while in its turn, the domestic perpetuates the cosmic. Her prose books were Din întuneric. Fapte și întâmplări adevărate. Din carnetul unei doctorese (1928), Grădina cu amintiri și alte schițe (1929), În târgușorul dintre vii... (1939); she also authored a novel, A murit Luchi... (1942). Some of these works include poetic sketches reminiscent of Antoine de Saint-Exupéry or Colette, while others are in a more realist vein. Cazimir worked as inspector-general of theaters in the Moldavia region from 1937 to 1947. She was involved in a discreet, years-long relationship with the married poet George Topîrceanu.