Depends on the context. If you are talking about electricity in California, it would be Independent System Operator. If you are talking about a company being ISO 9000 certified, it would be International Standards Organization. There are probably more but these I knew.
ISO - [not an acronym] common name for International Organization for Standardization;oddly enough, the world doesn't revolve around the English language...
ISO - In Search Of
ISO - Imamia Students Organization (Pakistan)
ISO - Imperial Service Order
ISO - Imperial Symphony Orchestra
ISO - In Support Of
ISO - Incentive Stock Option
ISO - Independent Sales Organization
ISO - Independent Sales Organization (Wang)
ISO - Independent System Operator
ISO - Independent System Operator (power grid)
ISO - Indianapolis Symphony Orchestra
ISO - Industry Standards Organization (IEEE-ISTO)
ISO - Information Systems Office
ISO - Information Systems Officer
ISO - Infrared Space Observatory
ISO - Installation Safety Office
ISO - Installation Supply Officer
ISO - Instead Of
ISO - Insurance Services Office, Inc.
ISO - Integration Staff Officer
ISO - Intelligence and Security Office(r)
ISO - International Satellite Organization
ISO - International Socialist Organization
ISO - International Society of Organbuilders
ISO - International Standards Organization (common, but incorrect)
ISO - International Student Organization
ISO - Internet Sales Outlet
ISO - Interservice Support Office
ISO - Isochronical Inspection (USAF; phase of aircraft maintenance)
ISO - Isolated Power (baseball statistic)
ISO - Isometric
ISO - Isotope
ISO - Istanbul Sanayi Odasi
ISO - Kinston, NC, USA (Airport Code)
ISO - Open System Interfacing