- OB OBligatory Joke
- OBI Online Book Initiative
- OBJ OBligatory Joke
- OC1 Optical Carrier Level 1
- OCIS Organized Crime Information Systems
- OCLC Online Computer Learning Center
- OCR Optical Character Recognition
- OCW Operation Command Word
- OCXs OLE Custom Controls (X stands for Control in this instance)
- ODA Open Document Architecture / Office Document Architecture
- ODAPI Open Database Application Programming Interface
- ODBC Open DataBase Connectivity
- ODI Open Data Link Interface
- ODMA Open Document Management API
- OE Output Enable
- OEM Original Equipment Manufacturer
- OF Overflow Flag
- OH Off Hook
- OIC Oh I See (C)
- OLAP On-Line Analytical Processing
- OLE Object Linking and Embedding
- OLTP OnLine Transaction Processing
- OMG Object Management Group
- OOBE Out Of Box Experience
- OOP Object Oriented Programming
- OOUI Object-Oriented User Interface
- OPC Organic Photoconducting Cartridge
- OPM Other People's Money
- ORAM Optical Random Access Memory
- ORB Object Request Broker
- OS Operating System
- OSDF Open Shortest Data First
- OSF Open Systems Function / Open Software Foundation
- OSI Open Systems Interconnection
- OSPF Open Shortest Path First
- OTF On The Floor
- OTOH On The Other Hand
- OTPROM One-Time Programmable Read-Only Memory
- OTTH On The Third Hand
- OUI Organizationally Unique Identifier
- OUTP OUTput to Port
- OWL Object Window Library