Count: 0 — There are no recognized vitamins that start with the letter O. Vitamins carry historic letter names (A, B, C, D, E, K) or specific chemical names for B-complex members, and authoritative bodies such as the NIH, WHO and EFSA do not list any essential vitamin whose accepted name begins with O. This gap reflects how the vitamin naming system evolved during early nutritional science rather than a missing nutritional category.

Understand why the search returns no results. Vitamins are defined by essentiality for normal growth or prevention of deficiency diseases; they were assigned letters as they were discovered, and later discoveries received chemical or descriptive names (for example, the multiple B vitamins). Many nutrient compounds do start with O — such as omega fatty acids, oleic acid, or orotic acid — but these are not classified as vitamins by major health authorities. Also, some commercial products label supplements with names like “vitamin O” for marketing, but those names do not correspond to a recognized, essential vitamin.

Check related categories when you need nutrients that begin with O. Look for omega-3 and omega-6 fatty acids (important fats), oleic acid (a common monounsaturated fat in olive oil), or phytochemicals like oligomeric proanthocyanidins; these provide health effects but sit outside the official vitamin list. For a complete, authoritative reference on vitamins, consult the recognized list (A, the B-complex vitamins, C, D2/D3, E, and K1/K2) and their RDIs from NIH, EFSA, WHO, or USDA FoodData Central.