IOI

IOI may refer to:
 * Idiopathic orbital inflammatory disease
 * International Olympiad in Informatics
 * Interonset interval
 * Indication of interest, used in financial trading
 * Interest on Investment, a synonym for Return on Investment (ROI)
 * IO Interactive, a Danish video game developer
 * International Ombudsman Institute
 * IOI Group, a Malaysian conglomerate
 * I.O.I, a South Korean girl group formed from the television show Produce 101
 * Innovative Online Industries (IOI), the fictional corporation that employs most of the antagonists of the 2011 novel Ready Player One and its 2018 film adaptation