| 2601 | OOST | OOSTDYK, ENGLE, INC | | | SCAC4 |
| 2602 | OOST | Oostnet | | NL | TRAFFIC |
| 2603 | OOT | Out of the Blue Air Safaris (PTY) Ltd. | Callsign: Ootbas, | ZA | AVCODES |
| 2604 | OOT | Out Of The Blue Air Safaris | | | ICAO1 |
| 2605 | OOTA | ONE ONE THREE ZERO ZERO | | | SCAC4 |
| 2606 | OOTG | OPTION ONE TRANSPORTATION GROUP LLC | | | SCAC4 |
| 2607 | OOTP | OPT OVERSEAS TRANSPORT | | | SCAC4 |
| 2608 | OOTU | OTLO SHIPPING LINE | | UAE | BIC |
| 2609 | OOTW | ONE ONE TWO (112) WELLINGTON STREET | | | SCAC4 |
| 2610 | OOUI | OWNER OPERATOR UNION INC | | | SCAC4 |
| 2611 | OOUU | ORIENT OVERSEAS LINE, INC. | | | PWRC |
| 2612 | OOUU | Orient Overseas Line | | | RSRV |
| 2613 | OOUU | ORIENT OVERSEAS CONTAINER LTD | | | SCAC3 |
| 2614 | OOUU | OCEANEERING INTERNATIONAL LIMITED | | | SCAC4 |
| 2615 | OOVL | ONE THOUSAND OAKS VAN LINES (1000 O | | | SCAC3 |
| 2616 | OOWH | OUR OWN HARDWARE CO | | | SCAC3 |
| 2617 | OOXL | ONTARIO-OHIO EXPRESS LTD | | | SCAC3 |
| 2618 | OOZN | ONE ONE ZERO NINE ONE SEVEN THREE ( | | | SCAC4 |
| 2619 | OP | Flying Boat Inc. d/b/a Chalk's International Airines | ICAO: CHK, Callsign: Chalks, IATA Prefix: 370, | USA | AVCODES |
| 2620 | OP | Flying Boat Inc. d/b/a Chalks International Airlines | ICAO: CHK, IATA Prefix: 370, | USA | AVCODES |
| 2621 | OP | Air Panama International | ICAO: API, Callsign: Arpa, | PA | AVCODES |
| 2622 | OP | Olympic Airlines | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, IATA designator changed from OA. Name changed from Olympic Airways | GR | AVCODES |
| 2623 | 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 |
| 2624 | OP | DAC International Airlines Ltd. d/b/a PassionAir | ICAO: DIG, Callsign: Dac Air, | GH | AVCODES |
| 2625 | OPA | Opal Air Pty Ltd. | | AUS | AVCODES |
| 2626 | OPA | Opal Air | | | ICAO1 |
| 2627 | OPAL | OPALOS Y | | | SCAC4 |
| 2628 | OPAT | OIL PATCH CORP | | | SCAC4 |
| 2629 | OPAX | Oxford Paper Co. | added 1/1959; eliminated 10/1959 | | NAKINA |
| 2630 | OPAX | Oxford Paper Co., Inc. | 10/1957; eliminated 1/1959 | | NAKINA |
| 2631 | OPAX | Oxford Paper Co. | added 4/1954; 7/1954 | | NAKINA |
| 2632 | OPAX | Oxford Paper Co. | added 4/54 | | NRHS |
| 2633 | OPAX | Oxford Paper Co., Inc. | 10/57; eliminated 1/59 | | NRHS |
| 2634 | OPAX | Oxford Paper Co. | added 1/59; eliminated 10/59 | | NRHS |
| 2635 | OPAX | OXFORD PAPER CO. | | | PWRC |
| 2636 | OPBI | OLYMPIC PACIFIC BUILDERS INC | | | SCAC3 |
| 2637 | OPBP | OREPAC BUILDING PRODUCTS | | | SCAC4 |
| 2638 | OPC | Krystel Air Charter Ltd. | Callsign: Optic, | GB | AVCODES |
| 2639 | OPC | Krystel Air Charter | | | ICAO1 |
| 2640 | OPCC | ONYX PAVING COMPANY INC | | | SCAC4 |
| 2641 | OPCI | OPTIMO CONSOLIDATORS INTERNATIONAL | | | SCAC4 |
| 2642 | OPCM | OSSIAN PACKING COMPANY | | | SCAC4 |
| 2643 | OPCO | OWENS PLATING CO | | | SCAC3 |
| 2644 | OPCO | OPSCO ENERGY INDUSTRIES USA LIMITED | | | SCAC4 |
| 2645 | OPCR | OPC TRUCKING | | | SCAC4 |
| 2646 | OPCS | OPTION COURIER SERVICES INC | | | SCAC3 |
| 2647 | OPCU | O P D R CANARIAS | | | SCAC4 |
| 2648 | OPCX | General Electric Railcar Services Corp. | added 7/1987; eliminated 1/1988 | | NAKINA |
| 2649 | OPCX | Permanente Cement Co. | added 7/1959; 4/1961; eliminated 10/1961 | | NAKINA |
| 2650 | OPCX | The Olympic Portland Cement Co., Ltd. | added 4/1954; 7/1954-10/1957; eliminated 7/1959 | | NAKINA |
| 2651 | OPCX | Fruit Growers Express Co. (reporting marks for Omaha Packing Co.) | 7/1937-1/1950 | | NAKINA |
| 2652 | OPCX | Omaha Packing Co. (National Car Co.) | Apr-38 | | NRHS |
| 2653 | OPCX | The Olympic Portland Cement Co., Ltd. | added 4/54; 10/57; eliminated 7/59 | | NRHS |
| 2654 | OPCX | Permanente Cement Co. | added 7/59; 4/61 | | NRHS |
| 2655 | OPCX | General Electric Railcar Services Corp. | added 7/87; eliminated 1/88 | | NRHS |
| 2656 | OPCX | *OMAHA PACKING CO. (FRUIT GROWERS EXPRESS) | | | PWRC |
| 2657 | OPCX | *PERMANENTE CEMENT CO. [OLYMPIC PORTLAND CEMENT CO. LTD.] | | | PWRC |
| 2658 | OPCX | *GENERAL ELECTRIC RAILCAR SERVICES CORP. | | | PWRC |
| 2659 | OPCY | OMNI PACIFIC COMPANY INC | | | SCAC4 |
| 2660 | OPDA | ONTARIO POTATO DIST (ALLISTON) INC | | | SCAC4 |
| 2661 | OPDL | OCEAN PETROLEUM DISTRIBUTORS L L C | | | SCAC4 |
| 2662 | OPDM | OLYMPIC PRODUCE DE MEXICO SA DE CV | | | SCAC4 |
| 2663 | OPDU | CMA-CGM | | F | BIC |
| 2664 | OPDU | OLDENBURG PORTUGIESISCHE | | | SCAC3 |
| 2665 | OPDU | OLDENBURG-PORTUGIESIC | | | SCAC4 |
| 2666 | OPDZ | OMAHA POWER DISTRICT | | | PWRC |
| 2667 | OPE | Société 3 S Aviation (Aerope) | Callsign: Pontoisair, | F | AVCODES |
| 2668 | 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 |
| 2669 | OPE | Oregon Pacific and Eastern Ry. Co. | 1/75-1/80; eliminated 1/94; eliminated 4/95; 7/2000 | | NRHS |
| 2670 | OPE | OREGON, PACIFIC & EASTERN RAILWAY COMPANY | | | PWRC |
| 2671 | OPE | Oregon, Pacific and Eastern Railway | | | RSRV |
| 2672 | OPE | OREGON PACIFIC & EASTERN RAILWAY CO | | | SCAC4 |
| 2673 | OPEG | OREGON, PACIFIC & EASTERN RAILWAY COMPANY | | | PWRC |
| 2674 | OPEI | OLIVER PIPE & EQUIPMENT INC | | | SCAC3 |
| 2675 | OPEL | OPTIMISTIC ENTERPRISES LTD | | | SCAC3 |
| 2676 | OPEL | | | D | TRAFFIC |
| 2677 | OPEN | OPEN WINDOW BAKERY LIMITED | | | SCAC4 |
| 2678 | OPEP | ORLY PLASTIC ENTERPRISES | | | SCAC4 |
| 2679 | OPFE | OCEAN PRIDE (FME) INC | | | SCAC4 |
| 2680 | OPFI | OCEAN PACIFIC FREIGHT INC | | | SCAC3 |
| 2681 | OPFI | OCEANIA PACIFIC FREIGHT INC | | | SCAC4 |
| 2682 | OPFL | OPAL FINANCE LTD | | | SCAC4 |
| 2683 | OPFS | OIL PATCH FUEL & SUPPLY INC | | | SCAC4 |
| 2684 | OPFW | OCEAN PROSPER INC | | | SCAC4 |
| 2685 | OPFX | Cia Occidental de Productos Forestales, S.A. | 1/1945-4/1947 | | NAKINA |
| 2686 | OPFX | CIA OCCIDENTAL DE PRODUCTOS FORESTALES, SA | | | PWRC |
| 2687 | OPGN | ONTARIO POWER GENERATION | | | SCAC4 |
| 2688 | OPGU | OGDEN POLAR GROUP | | USA | BIC |
| 2689 | OPGX | Oklahoma Petroleum & Gasoline Co. | 8/1920 | | NAKINA |
| 2690 | OPGX | OKLAHOMA PETROLEUM & GASOLINE CO. | | | PWRC |
| 2691 | OPHG | OPPENHEIME | | | SCAC4 |
| 2692 | OPHI | OPHIR TOURS PTY LTD | | | SCAC3 |
| 2693 | OPHP | OIL PATCH PETROLEUM INC | | | SCAC4 |
| 2694 | OPIC | OCEAN PETROLEUM INC | | | SCAC4 |
| 2695 | OPIL | ONSTREAM PIPELINE INSPECTION LTD | | | SCAC4 |
| 2696 | OPIN | OAKCREEK PROPANE INC | | | SCAC4 |
| 2697 | OPIX | Equistar Chemicals, LP | added 10/1998; 7/2000-7/2005 | | NAKINA |
| 2698 | OPIX | Occidental Chemical Corp. | added 1/1992; 1/1994; eliminated 10/1998 | | NAKINA |
| 2699 | OPIX | Occidental Chemical Corp., Petrochemicals Division | added 4/1991; eliminated 1/1992 | | NAKINA |
| 2700 | OPIX | OXY Petrochemicals Inc. | eliminated 4/1991 | | NAKINA |
