Organization of American States Offensive Air Support Office of Applied Studies Offensive Avionics System Old Age Security Obstacle assessment surface Office Automation System Office of Aircraft Services One-to-All Scatter Option Adjusted Spread Orbiter Aeroflight Simulator Orbiter Atmospheric Simulator Orbiter Avionics System Office of Administrative Services Open Application Server Organisation of American States Obstacle Assessment Surfaces Office of the Assistant Secretary Online Assessment System Operational Areas Operations Applications optical alignment sights Oracle Application Server Oral allergy syndrome Orbit Analysis System Orbital Analysis System Orbiter Attachment System Orbiter Avionics Simulator O-acetyl-L-serine O-acetylserine oceanic automation system Offensive Avionics Suite Offfice of Applied Studies Office Administration Services Office Alarm System Office Automation Software Office Automation Subcommittee Office Automation Systems Office of Accounting Services Office of Aerospace Studies