Oin, Óin or OIN may refer to:
Folklore and fiction Óin, son of Gróin, a Dwarf from J. R. R. Tolkien's writings, companion of Thorin Oakenshield Oin-Oin, a character of Swiss folklore
Other Open Invention Network, a company specialising in Linux patents Oneida Indian Nation, the New York tribe that operates Turning Stone Resort & Casino One language, specifically Inebu, by ISO 639-3 code
See also Eoin, an Irish name pronounced O-in