Archaic noun for the mandarin duck. In modern Japanese, オシドリ (oshidori) is the standard word.
In old waka poetry, 鴛鴦 is sometimes read as おし.
The historical derivation is uncertain. The kanji 鴛鴦 are Chinese characters for the mandarin duck, and the reading おし is a jukujikun (special kanji reading) that predates the modern オシドリ.