| Acronym | Definition |
|---|
| IO | Input/Output |
| IO | Iowa (old style) |
| IO | Information Operations |
| IO | Indian Ocean |
| IO | Interest Only |
| IO | Igrzyska Olimpijskie (Polish: Olympic Games) |
| IO | Inside Out (Frisbee throw) |
| IO | Industrial-Organizational (Psychology) |
| IO | British Indian Ocean Territory |
| IO | If Only |
| IO | International Organization |
| IO | Information Object |
| IO | Information Officer |
| IO | Information Overload |
| IO | Infrastructure Optimization |
| IO | Indirect Object |
| IO | Intraosseous |
| IO | Intermon Oxfam (Spain) |
| IO | Intelligence Officer |
| IO | In Organization (banking) |
| IO | Investigating Officer (law enforcement) |
| IO | Industrial Operations |
| IO | Information Output |
| IO | Internal Order |
| IO | Institutional Official (various organizations) |
| IO | Imperial Order (gaming) |
| IO | Ionium (Thorium Isotope) |
| IO | Insertion Order (web advertising) |
| IO | Illumninati Order |
| IO | Internal Organ |
| IO | Intercept Officer (US Navy) |
| IO | Institute of Oceanography |
| IO | Integrated Operations (oil industry) |
| IO | Interactive Optimum (cable services) |
| IO | Indian Orchard (Springfield, MA) |
| IO | Immigration Officer |
| IO | Intelligence Oversight |
| IO | Integrated Optics |
| IO | Illuminati Online |
| IO | Information Orientation |
| IO | Independent Observations |
| IO | Immediate Office |
| IO | Inboard Outboard (boat drive system) |
| IO | Input Operator |
| IO | International Officer |
| IO | Infinitely Often |
| IO | Intermediate Objective |
| IO | Inspecciòn Ocular (Guatemala, police task force) |
| IO | Insignificant Other |
| IO | Initial Only |
| IO | iodine monoxide |
| IO | Information Outlet (LAN) |
| IO | Inexperienced Operator |
| IO | Inspectie van het Onderwijs (Dutch) |
| IO | Instant Outlining |
| IO | Installing Officer (International Order of DeMolay) |
| IO | Inventory Objective |
| IO | Idiot Operator |
| IO | InvestOne (software) |
| IO | Isomerized Olefin |
| IO | Interoperability Objective |
| IO | Individually Optimum |