Operational Intercommunication System 
Office of Information Services 
Office Information System 
Office Information Systems 
Operational Intercom System 
Operational Information System 
opportunistic infections 
Orbiter Instrumentation Systems 
Ordnance Information System 
Objective Instrumentation System 
Objective Interface Systems 
Office of Information Security 
Office of Information Systems 
Officer Indoctrination School 
Oh I see 
Open Interface Specification 
operating information system 
Operational Information Service 
Optical Imaging System 
Orbiter Insertion Stage 
Object Identification System 
Obstacle Identification Surface 
Obstruction Identification Surfaces 
ocular ischemic syndrome 
Office of Instructional Services 
Office of International Services 
Office of Investigatory Services 
OGE INPUT SIMULATOR 
Oil States International, Inc. 
Ontario Investment Service