| 1 | OP | Flying Boat Inc. d/b/a Chalk's International Airines | ICAO: CHK, Callsign: Chalks, IATA Prefix: 370, | USA | AVCODES |
| 2 | OP | Flying Boat Inc. d/b/a Chalks International Airlines | ICAO: CHK, IATA Prefix: 370, | USA | AVCODES |
| 3 | OP | Air Panama International | ICAO: API, Callsign: Arpa, | PA | AVCODES |
| 4 | OP | Olympic Airlines | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, IATA designator changed from OA. Name changed from Olympic Airways | GR | AVCODES |
| 5 | OP | Air Pegasus Pvt. Ltd. | ICAO: PPL, Callsign: Pegasus, Founded in 2014. Commenced operations on 12 April 2015. Ceased operations on 27 July 2016. | IND | AVCODES |
| 6 | OP | DAC International Airlines Ltd. d/b/a PassionAir | ICAO: DIG, Callsign: Dac Air, | GH | AVCODES |
| 7 | OPA | Opal Air Pty Ltd. | | AUS | AVCODES |
| 8 | OPA | Opal Air | | | ICAO1 |
| 9 | OPAL | OPALOS Y | | | SCAC4 |
| 10 | OPAT | OIL PATCH CORP | | | SCAC4 |
| 11 | OPAX | Oxford Paper Co. | added 1/1959; eliminated 10/1959 | | NAKINA |
| 12 | OPAX | Oxford Paper Co., Inc. | 10/1957; eliminated 1/1959 | | NAKINA |
| 13 | OPAX | Oxford Paper Co. | added 4/1954; 7/1954 | | NAKINA |
| 14 | OPAX | Oxford Paper Co. | added 4/54 | | NRHS |
| 15 | OPAX | Oxford Paper Co., Inc. | 10/57; eliminated 1/59 | | NRHS |
| 16 | OPAX | Oxford Paper Co. | added 1/59; eliminated 10/59 | | NRHS |
| 17 | OPAX | OXFORD PAPER CO. | | | PWRC |
| 18 | OPBI | OLYMPIC PACIFIC BUILDERS INC | | | SCAC3 |
| 19 | OPBP | OREPAC BUILDING PRODUCTS | | | SCAC4 |
| 20 | OPC | Krystel Air Charter Ltd. | Callsign: Optic, | GB | AVCODES |
| 21 | OPC | Krystel Air Charter | | | ICAO1 |
| 22 | OPCC | ONYX PAVING COMPANY INC | | | SCAC4 |
| 23 | OPCI | OPTIMO CONSOLIDATORS INTERNATIONAL | | | SCAC4 |
| 24 | OPCM | OSSIAN PACKING COMPANY | | | SCAC4 |
| 25 | OPCO | OWENS PLATING CO | | | SCAC3 |
| 26 | OPCO | OPSCO ENERGY INDUSTRIES USA LIMITED | | | SCAC4 |
| 27 | OPCR | OPC TRUCKING | | | SCAC4 |
| 28 | OPCS | OPTION COURIER SERVICES INC | | | SCAC3 |
| 29 | OPCU | O P D R CANARIAS | | | SCAC4 |
| 30 | OPCX | General Electric Railcar Services Corp. | added 7/1987; eliminated 1/1988 | | NAKINA |
| 31 | OPCX | Permanente Cement Co. | added 7/1959; 4/1961; eliminated 10/1961 | | NAKINA |
| 32 | OPCX | The Olympic Portland Cement Co., Ltd. | added 4/1954; 7/1954-10/1957; eliminated 7/1959 | | NAKINA |
| 33 | OPCX | Fruit Growers Express Co. (reporting marks for Omaha Packing Co.) | 7/1937-1/1950 | | NAKINA |
| 34 | OPCX | Omaha Packing Co. (National Car Co.) | Apr-38 | | NRHS |
| 35 | OPCX | The Olympic Portland Cement Co., Ltd. | added 4/54; 10/57; eliminated 7/59 | | NRHS |
| 36 | OPCX | Permanente Cement Co. | added 7/59; 4/61 | | NRHS |
| 37 | OPCX | General Electric Railcar Services Corp. | added 7/87; eliminated 1/88 | | NRHS |
| 38 | OPCX | *OMAHA PACKING CO. (FRUIT GROWERS EXPRESS) | | | PWRC |
| 39 | OPCX | *PERMANENTE CEMENT CO. [OLYMPIC PORTLAND CEMENT CO. LTD.] | | | PWRC |
| 40 | OPCX | *GENERAL ELECTRIC RAILCAR SERVICES CORP. | | | PWRC |
| 41 | OPCY | OMNI PACIFIC COMPANY INC | | | SCAC4 |
| 42 | OPDA | ONTARIO POTATO DIST (ALLISTON) INC | | | SCAC4 |
| 43 | OPDL | OCEAN PETROLEUM DISTRIBUTORS L L C | | | SCAC4 |
| 44 | OPDM | OLYMPIC PRODUCE DE MEXICO SA DE CV | | | SCAC4 |
| 45 | OPDU | CMA-CGM | | F | BIC |
| 46 | OPDU | OLDENBURG PORTUGIESISCHE | | | SCAC3 |
| 47 | OPDU | OLDENBURG-PORTUGIESIC | | | SCAC4 |
| 48 | OPDZ | OMAHA POWER DISTRICT | | | PWRC |
| 49 | OPE | Société 3 S Aviation (Aerope) | Callsign: Pontoisair, | F | AVCODES |
| 50 | OPE | Oregon Pacific and Eastern Ry. Co. | 1/1975-1/1980; eliminated 1/1994; 1/1994 (not listed); eliminated 4/1995; 7/2000-7/2005 | | NAKINA |
| 51 | OPE | Oregon Pacific and Eastern Ry. Co. | 1/75-1/80; eliminated 1/94; eliminated 4/95; 7/2000 | | NRHS |
| 52 | OPE | OREGON, PACIFIC & EASTERN RAILWAY COMPANY | | | PWRC |
| 53 | OPE | Oregon, Pacific and Eastern Railway | | | RSRV |
| 54 | OPE | OREGON PACIFIC & EASTERN RAILWAY CO | | | SCAC4 |
| 55 | OPEG | OREGON, PACIFIC & EASTERN RAILWAY COMPANY | | | PWRC |
| 56 | OPEI | OLIVER PIPE & EQUIPMENT INC | | | SCAC3 |
| 57 | OPEL | OPTIMISTIC ENTERPRISES LTD | | | SCAC3 |
| 58 | OPEL | | | D | TRAFFIC |
| 59 | OPEN | OPEN WINDOW BAKERY LIMITED | | | SCAC4 |
| 60 | OPEP | ORLY PLASTIC ENTERPRISES | | | SCAC4 |
| 61 | OPFE | OCEAN PRIDE (FME) INC | | | SCAC4 |
| 62 | OPFI | OCEAN PACIFIC FREIGHT INC | | | SCAC3 |
| 63 | OPFI | OCEANIA PACIFIC FREIGHT INC | | | SCAC4 |
| 64 | OPFL | OPAL FINANCE LTD | | | SCAC4 |
| 65 | OPFS | OIL PATCH FUEL & SUPPLY INC | | | SCAC4 |
| 66 | OPFW | OCEAN PROSPER INC | | | SCAC4 |
| 67 | OPFX | Cia Occidental de Productos Forestales, S.A. | 1/1945-4/1947 | | NAKINA |
| 68 | OPFX | CIA OCCIDENTAL DE PRODUCTOS FORESTALES, SA | | | PWRC |
| 69 | OPGN | ONTARIO POWER GENERATION | | | SCAC4 |
| 70 | OPGU | OGDEN POLAR GROUP | | USA | BIC |
| 71 | OPGX | Oklahoma Petroleum & Gasoline Co. | 8/1920 | | NAKINA |
| 72 | OPGX | OKLAHOMA PETROLEUM & GASOLINE CO. | | | PWRC |
| 73 | OPHG | OPPENHEIME | | | SCAC4 |
| 74 | OPHI | OPHIR TOURS PTY LTD | | | SCAC3 |
| 75 | OPHP | OIL PATCH PETROLEUM INC | | | SCAC4 |
| 76 | OPIC | OCEAN PETROLEUM INC | | | SCAC4 |
| 77 | OPIL | ONSTREAM PIPELINE INSPECTION LTD | | | SCAC4 |
| 78 | OPIN | OAKCREEK PROPANE INC | | | SCAC4 |
| 79 | OPIX | Equistar Chemicals, LP | added 10/1998; 7/2000-7/2005 | | NAKINA |
| 80 | OPIX | Occidental Chemical Corp. | added 1/1992; 1/1994; eliminated 10/1998 | | NAKINA |
| 81 | OPIX | Occidental Chemical Corp., Petrochemicals Division | added 4/1991; eliminated 1/1992 | | NAKINA |
| 82 | OPIX | OXY Petrochemicals Inc. | eliminated 4/1991 | | NAKINA |
| 83 | OPIX | OXY USA Inc. | added 1/1991 | | NAKINA |
| 84 | OPIX | OXY NGL, Inc. | eliminated 1/1991 | | NAKINA |
| 85 | OPIX | OXY Petrochemicals Inc. | added 7/1990 | | NAKINA |
| 86 | OPIX | OXY Petrochemicals Inc. | added 7/90 | | NRHS |
| 87 | OPIX | OXY NGL, Inc. | eliminated 1/91 | | NRHS |
| 88 | OPIX | OXY USA Inc. | added 1/91 | | NRHS |
| 89 | OPIX | OXY Petrochemicals Inc. | eliminated 4/91 | | NRHS |
| 90 | OPIX | Occidental Chemical Corp., Petrochemicals Division | added 4/91; eliminated 1/92 | | NRHS |
| 91 | OPIX | Occidental Chemical Corp. | added 1/92; 1/94; eliminated 10/98 | | NRHS |
| 92 | OPIX | Equistar Chemicals, LP | added 10/98; 7/2000 | | NRHS |
| 93 | OPIX | EQUISTAR CHEMICALS, LP [OCCIDENTAL CHEMICAL CORP., - PETROCHEMICALS DIVISION; OXY USA, INC.; OXY NGL, INC.; OXY PETROCHEMICALS] | | | PWRC |
| 94 | OPIX | Equistar Chemicals | | | RSRV |
| 95 | OPJ | Opera Jet s.r.o. | Callsign: Opera Jet, | SK | AVCODES |
| 96 | OPKM | OPTIKAM TECH INC | | | SCAC4 |
| 97 | OPKS | PKS Opole | | PL | TRAFFIC |
| 98 | OPKT | OVERPECK TRANSPORT CORP | | | SCAC4 |
| 99 | OPKX | Onalaska Pickle & Canning Co. | 7/1935 | | NAKINA |
| 100 | OPL | Air Cote D'opale, Sàrl | | F | AVCODES |
