The term "oneshota" is a portmanteau of "oneesan" (older sister) and "shota" (young boy). While often associated with mature content, the broader genre covers a wide range of storytelling styles, from sweet slice-of-life romances to more explicit fantasy scenarios like those seen in this specific 2020 adaptation. Oneshota the Animation (Video 2020) - IMDb
The series is typically categorized within the "shota" subgenre, which focuses on young male protagonists. As with many titles in this category, it is often produced as a Short Original Video Animation (OVA) based on existing manga or dōjinshi.
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To understand the title, you have to break down the Japanese terminology. The term "Oneshota" is a portmanteau of two words: Meaning "older sister" or "young lady." A term for a young or young-looking boy character.