| 101 | OAKW | OAKWOOD TRUCKING CORP | | | SCAC3 |
| 102 | OAKY | OAKRIDGE TRUCKING | | | SCAC4 |
| 103 | OAL | Olympic Airways | IATA: OA, Callsign: Olympic, IATA Prefix: 050, Name changed to Olympic Airlines | GR | AVCODES |
| 104 | OAL | Olympic Airlines | IATA: OA, Callsign: Olympic, IATA Prefix: 050, IATA designator changed to OP. Name changed from Olympic Airways | GR | AVCODES |
| 105 | OAL | Olympic Airlines | IATA: OP, Callsign: Olympic, IATA Prefix: 050, IATA designator changed from OA. Name changed from Olympic Airways | GR | AVCODES |
| 106 | OAL | Olympic Air | IATA: OA, Callsign: Olympic, IATA Prefix: 050, | GR | AVCODES |
| 107 | OAL | Olympic Airlines | IATA: OA | | ICAO1 |
| 108 | OALU | OCEAN AFRICA CONTAINER LINE PTY LTD | | | SCAC4 |
| 109 | OALX | Oregon-American Lumber Corp. | 7/1937-1/1952; eliminated 1/1953 | | NAKINA |
| 110 | OALX | Oregon-American Lumber Corp. | 4/38-1/52; eliminated 1/53 | | NRHS |
| 111 | OALX | OREGON-AMERICAN LUMBER CORP. | | | PWRC |
| 112 | OAMX | Omaha Alfalfa Milling Co. | 8/1920-7/1923 | | NAKINA |
| 113 | OAMX | OMAHA ALFALFA MILLING CO. | | | PWRC |
| 114 | OAN | Aerospace One S.A. | IATA: IW, Callsign: Hellas Cargo, IATA Prefix: 271, | GR | AVCODES |
| 115 | OAN | NATO North Atlantic Treaty Organisation | Callsign: OTAN, | B | AVCODES |
| 116 | OANI | OCEANICA PRODUCTS | | | SCAC4 |
| 117 | OAO | Arkhangelsk Airlines | IATA: 5N, Callsign: Dvina, IATA Prefix: 316, Name changed to Aeroflot - Nord. ICAO codes changed to AUL. IATA Designator changed from 5N* | RUS | AVCODES |
| 118 | OAO | Arkhangelsk Airlines | IATA: 5N, Callsign: Dvina, IATA Designator changed to 5N | RUS | AVCODES |
| 119 | OAO | Arkhangelsk 2 Aviation Division | | | ICAO1 |
| 120 | OAP | Coapa Air, S.A. de C.V. | Callsign: Coapa, | MEX | AVCODES |
| 121 | OAP | COAPA AIR | | | ICAO1 |
| 122 | OAQC | OMEGA-3 AQUACULTURE | | | SCAC4 |
| 123 | OAR | Grupo One Air Aviacion, S.L. | Callsign: Boss Air, | E | AVCODES |
| 124 | OAR | Old Augusta R.R. Co. | added 7/1983; 1/1994-7/2005; eliminated 7/2006 | | NAKINA |
| 125 | OAR | Old Augusta R.R. LLC | added 7/2006 | | NAKINA |
| 126 | OAR | Old Augusta R.R. Co. | 1/94-7/2000 | | NRHS |
| 127 | OAR | OLD AUGUSTA RAILROAD COMPANY | | | PWRC |
| 128 | OAR | Old Augusta Railroad | | | RSRV |
| 129 | OAR | OLD AUGUSTA RAILROAD LLC | | | SCAC4 |
| 130 | OARD | ORCHARD PARK GROWERS LTD | | | SCAC4 |
| 131 | OARK | OLD ARK TRUCKING LLC | | | SCAC4 |
| 132 | OAS | Oman Aviation Services Co. (SAOG) | IATA: WY, Callsign: Oryx, IATA Prefix: 910, Name changed to Oman Air (SAOG). ICAO callsign changed from Whiskey Yankee | OM | AVCODES |
| 133 | OAS | Oman Air (SAOG) | IATA: WY, Callsign: Oryx, IATA Prefix: 910, Name changed from Oman Aviation Services Co. (SAOG) | OM | AVCODES |
| 134 | OAS | Oasis Flight Ltd | | GB | AVCODES |
| 135 | OAS | Oman Aviation Services Co. (SAOG) | IATA: WY, Callsign: Whiskey Yankee, IATA Prefix: 910, Name changed to Oman Air (SAOG). ICAO callsign changed to Oryx | OM | AVCODES |
| 136 | OASB | ORIENT ALLIANCE SHIPPING COMPANY LI | | | SCAC4 |
| 137 | OASC | OASIS CARGO | | | SCAC3 |
| 138 | OASC | OCEAN ANGEL SHIPPING CO | | | SCAC4 |
| 139 | OASD | OASIS DENTAL GROUP INC | | | SCAC4 |
| 140 | OASE | OASIS ENTERPRISE INC | | | SCAC3 |
| 141 | OASI | OASIS COMPANY INC | | | SCAC3 |
| 142 | OASI | OASIS ENE | | | SCAC4 |
| 143 | OASK | OCEGUERA AND SONS TRUCKING | | | SCAC4 |
| 144 | OASL | OASIS ALIGNMENT SERVICES LTD | | | SCAC4 |
| 145 | OASS | OASIS LINES INC | | | SCAC3 |
| 146 | OASU | OCEAN AXIS SHIPPING SERVICES LLC | | UAE | BIC |
| 147 | OAT | Omni Air Transport, LLC | Callsign: Driller, ICAO Designator changed to DRL | USA | AVCODES |
| 148 | OATC | OLMSTEAD, ARCHIE TRUCK CO | | | SCAC3 |
| 149 | OATD | OATES, DAVID | | | SCAC3 |
| 150 | OATE | OATES BROS, INC | | | SCAC3 |
| 151 | OATE | OATES BROTHERS TRUCKING INC | | | SCAC4 |
| 152 | OATG | OLVERA TRUCKING | | | SCAC4 |
| 153 | OATI | OMEGA TRANSPORTATION INC | | | SCAC3 |
| 154 | OATJ | ORIENTAL AIR TRANSPORT SERVICE INC | | | SCAC4 |
| 155 | OATN | OCEAN AIR TRANSPORTATION INC | | | SCAC3 |
| 156 | OATP | OAT ENTERPRISES INC | | | SCAC3 |
| 157 | OATR | OCEANSIDE AUTO TRANS INC | | | SCAC3 |
| 158 | OAU | Oriol Avia | Callsign: Oravia, | RUS | AVCODES |
| 159 | OAV | Omni - Aviacao e Tecnologia | | | ICAO1 |
| 160 | OAW | Odette Airways AG | Callsign: Arabella, Name changed to Helvetic Airways AG | CH | AVCODES |
| 161 | OAW | Helvetic Airways AG | IATA: 2L, Callsign: Arabella, ICAO Callsign changed to Helvetic. Name changed from Odette Airways AG. | CH | AVCODES |
| 162 | OAW | Helvetic Airways AG | IATA: 2L, Callsign: Helvetic, ICAO Callsign changed from Arabella. Name changed from Odette Airways AG. | CH | AVCODES |
| 163 | OAW | Helvetic Airways | IATA: 2L | | ICAO1 |
| 164 | OAX | Operational Aviation Services | | | ICAO1 |
| 165 | OAZ | Odessa Aircraft Plant | | UA | AVCODES |
| 166 | OB | Astrakhan Airlines | ICAO: ASZ, Callsign: Air Astrakhan, IATA Prefix: 562, Designator OB recalled by IATA | RUS | AVCODES |
| 167 | OB | Shepparton Airlines | Designator recalled by IATA | AUS | AVCODES |
| 168 | OB | Oasis International Airlines | ICAO: AAN, Callsign: Oasis, Ceased operations in December 1996 | E | AVCODES |
| 169 | OB | Austrian Airtransport (Osterreichische Flugbetriebsgesellschaft MbH.) | ICAO: AAT, Callsign: Austrian Charter, ICAO designator changed from AUC | A | AVCODES |
| 170 | OB | Austrian Airtransport (Osterreiechische Flugbetriebsgesellschaft mbH.) | ICAO: AUC, Callsign: Aircharter, ICAO callsign changed to Austrain Aircharter. Charter subsidary of Austrian Airlines | A | AVCODES |
| 171 | OB | Austrian Airtransport (Osterreiechische Flugbetriebsgesellschaft mbH.) | ICAO: AUC, Callsign: Austrian charter, ICAO callsign changed from Aircharter. Charter subsidary of Austrian Airlines | A | AVCODES |
| 172 | OB | Air Bee S.p.A. | ICAO: CMM, | I | AVCODES |
| 173 | OB | Boliviana de Aviacion (BoA) | ICAO: BOV, Callsign: Boliviana, IATA Prefix: 930, Founded on 24 October 2007 | BOL | AVCODES |
| 174 | OBA | Aerobanana, S.A. de C.V. | Callsign: Aerobanana, | MEX | AVCODES |
| 175 | OBA | Aerobanana | | | ICAO1 |
| 176 | OBA | Ofotbanen | | N | TRAFFIC |
| 177 | OBAG | OASIS BAGS | | | SCAC4 |
| 178 | OBAQ | OBA PRODUCE COMPANY | | | SCAC3 |
| 179 | OBAQ | OBC NEDERLAND LIMITED | | | SCAC4 |
| 180 | OBAR | OBARA & BREHM TRANSFER INC | | | SCAC4 |
| 181 | OBAS | OBRIEN & SMITH TRUCKING CO | | | SCAC3 |
| 182 | OBAS | Ofotbanen | | NO | VKM |
| 183 | OBB | Österreichische Budesbahn | | A | TRAFFIC |
| 184 | OBB | Österreichische Bundesbahnen | | AT | VKM |
| 185 | OBBL | Orange & Black Bus Lines | | USA | TRAFFIC |
| 186 | OBBP | ÖBB Postbus | | A | TRAFFIC |
| 187 | OBBPB | Österreichische Postbus AG | | A | TRAFFIC |
| 188 | OBBT | OTOS, BILL BOAT TRAILERING (HAULING | | | SCAC3 |
| 189 | OBC | Olympic Broadcasting Services | | GB | AVCODES |
| 190 | OBCC | OLENICK BROS COAL CO | | | SCAC3 |
| 191 | OBCC | O B C NORTHWEST INC | | | SCAC4 |
| 192 | OBCG | O'BRIEN CARTAGE CO | | | SCAC3 |
| 193 | OBCG | OBRIEN CARTAGE CO | | | SCAC4 |
| 194 | OBCI | OCEAN BULK CARRIERS INC, | | | SCAC3 |
| 195 | OBCI | OCEANIC BULK CARRIERS LTD | | | SCAC4 |
| 196 | OBCU | N.V. COBO CONSTRUCTION | | SME | BIC |
| 197 | OBCU | ORBICON | | | SCAC3 |
| 198 | OBCU | ORBICON | | | SCAC4 |
| 199 | OBCX | The Neatsfoot Oil Refineries Corp. | 7/1935-4/1938 | | NAKINA |
| 200 | OBCX | The Neatsfoot Oil Refineries Corp. | Apr-38 | | NRHS |
