| 1 | OK | Czech Airlines A.S. , CSA | ICAO: CSA, Callsign: CSA-Lines, IATA Prefix: 064, | CZ | AVCODES |
| 2 | OK | Ceskoslovenske Aerolinie (CSA) | ICAO: CSA, Callsign: CSA-Lines, IATA Prefix: 064, | CS | AVCODES |
| 3 | OK | Oklahoma Ry. Co. | 7/1923-1/1941 | | NAKINA |
| 4 | OK | Oklahoma Ry. Co. | Apr-38 | | NRHS |
| 5 | OK | OKLAHOMA RAILWAY CO. | | | PWRC |
| 6 | OK | O&K | | B | TRAFFIC |
| 7 | OKA | Okay Airways Company Limited | IATA: BK, Callsign: Okayjet, IATA Prefix: 866, Designator BK* reinstated by IATA | PRC | AVCODES |
| 8 | OKA | Okay Airways | | | ICAO1 |
| 9 | OKAM | OKAMI INC | | | SCAC4 |
| 10 | OKAN | Okanagan Valley Ry. Co. | 7/2000-7/2005 | | NAKINA |
| 11 | OKAN | Okanagan Valley Ry. Co. | Jul-00 | | NRHS |
| 12 | OKAN | OKANAGAN VALLEY RAILWAY (OMNITRAX) | | | PWRC |
| 13 | OKAN | Okanagan Valley Railway (OmniTRAX) | | | RSRV |
| 14 | OKAN | OKANAGAN VALLEY RAILWAY COMPANY | | | SCAC4 |
| 15 | OKAS | OKANOGAN-SEATTLE TRANSPORT COMPANY, | | | SCAC3 |
| 16 | OKAT | OKA TRANSFER CO | | | SCAC3 |
| 17 | OKAT | OKA TRANSFER CO, INC | | | SCAC4 |
| 18 | OKAV | OAKLANE COLONY | | | SCAC4 |
| 19 | OKAYAMA | Okayama Electric Tramway | City: Okayama | J | TRAFFIC |
| 20 | OKB | Ostkustbanan | 1923 - 1933 | S | svenska-lok.se |
| 21 | OKB | Sällskapet Ostkustbanans vänner | | SE | VKM |
| 22 | OKBQ | OKEEFE, BINYON MOVING & STORAGE INC | | | SCAC3 |
| 23 | OKBR | OAKLEY BARGE LINE INC | | | SCAC3 |
| 24 | OKBR | OAKLEY BROTHERS TRUCKING | | | SCAC4 |
| 25 | OKC | CZ Airlines, J.S.C. | | CZ | AVCODES |
| 26 | OKC | Private Jets, Inc. | Callsign: Thunder Wings, | USA | AVCODES |
| 27 | OKCC | OK'S CASCADE CO | | | SCAC3 |
| 28 | OKCC | OMEGA TRUCKLINES INC | | | SCAC4 |
| 29 | OKCE | Okarche Central Ry., Inc. | added 10/1982; eliminated 1/1983 | | NAKINA |
| 30 | OKCE | Okarche Central Ry., Inc. | added 10/82; eliminated 1/83 | | NRHS |
| 31 | OKCE | OKARCHE CENTRAL RAILWAY | | | PWRC |
| 32 | OKCE | Okarche Central Railway | | | RSRV |
| 33 | OKCG | OAK CARTAGE INC | | | SCAC3 |
| 34 | OKCR | OKLAHOMA CITY RELOAD | | | SCAC4 |
| 35 | OKCT | OKARCHE CENTRAL RAILWAY | | | PWRC |
| 36 | OKCU | CAHAYA SAMUDERA SHIPPING PTE LTD | | | SCAC4 |
| 37 | OKCX | Lone Star Industries, Inc. | added 7/1983; 1/1994-7/2000; eliminated 1/2001; added 7/2001; 7/2005 | | NAKINA |
| 38 | OKCX | Louisiana Cement Co., Inc. | added 10/1981; eliminated 7/1983 | | NAKINA |
| 39 | OKCX | OKC Corp. | added 4/1980; eliminated 10/1981 | | NAKINA |
| 40 | OKCX | OKC Corp. | added 4/80; eliminated 10/81 | | NRHS |
| 41 | OKCX | Louisiana Cement Co., Inc. | added 10/81 | | NRHS |
| 42 | OKCX | Lone Star Industries, Inc. | 1/94-7/2000; eliminated 1/2001; added 7/2001 | | NRHS |
| 43 | OKCX | LONE STAR INDUSTRIES, INC. [LOUISIANA CEMENT CO.; OKC CORP.] | | | PWRC |
| 44 | OKCX | Lone Star Industries | | | RSRV |
| 45 | OKCX | LONE STAR INDUSTRIES, INC. | | | SCAC4 |
| 46 | OKD | | | CZ | TRAFFIC |
| 47 | OKDB | OKAY DISTRIBUTORS | | | SCAC3 |
| 48 | OKDC | OK DISTRIBUTION INC | | | SCAC3 |
| 49 | OKDC | OAKPORT TRANSPORTATION SERVICES INC | | | SCAC4 |
| 50 | OKDD | OKD, Doprava, a.s. | | CZ | VKM |
| 51 | OKDI | OAKRIDGE DISTRIBUTION INC | | | SCAC3 |
| 52 | OKDJ | Odense - Kerteminde - Dalby Jernbane (Kertemindebanen) | 1900 - 1914 | DK | jernbanen.dk |
| 53 | OKDJ | Odense - Kerteminde - Dalby Jernbane (Kertemindebanen) | 1900 - 1914 | DK | TRAFFIC |
| 54 | OKDV | OKELLEY, DAVE CO INC | | | SCAC3 |
| 55 | OKEA | OKEANOPOROS CORP | | | SCAC3 |
| 56 | OKEE | OKEEFE, TERRANCE J | | | SCAC3 |
| 57 | OKEL | OK EXPRESS LTD | | | SCAC3 |
| 58 | OKEN | OHIO KENTUCKY STEEL CORPORATION | | | SCAC4 |
| 59 | OKEQ | OKEEFE, CASEY | | | SCAC3 |
| 60 | OKET | OKAPI EXPRESS TRUCKING | | | SCAC4 |
| 61 | OKEX | OKEX, Inc. | added 7/1976; 1/1980; eliminated 7/1982 | | NAKINA |
| 62 | OKEX | Okee, Inc. | added 7/1971; 1/1975; eliminated 7/1976 | | NAKINA |
| 63 | OKEX | Okee, Inc. | Jan-75 | | NRHS |
| 64 | OKEX | OKEX, Inc. | 1/80; eliminated 7/82 | | NRHS |
| 65 | OKEX | OKEX, INC. (OKEE, INC.) | | | PWRC |
| 66 | OKFD | ORVAL KENT FOOD COMPANY INC | | | SCAC4 |
| 67 | OKFE | OKEEFE, RANDY TRUCKING | | | SCAC3 |
| 68 | OKFE | OKEEFE, RANDY TRUCKING | | | SCAC4 |
| 69 | OKFF | OKLAHOMA FAST FREIGHT INC | | | SCAC3 |
| 70 | OKFF | OKLAHOMA FAST FREIGHT INC | | | SCAC4 |
| 71 | OKFQ | O K FAST FREIGHT | | | SCAC3 |
| 72 | OKGE | OKLAHOMA GENERAL ELECTRIC | | | PWRC |
| 73 | OKGN | OAKRIDGE GREENHOUSE | | | SCAC4 |
| 74 | OKIM | Okinawa Urban Monorail | | J | TRAFFIC |
| 75 | OKIN | OKLAHOMA INTERSTATE LINES INC | | | SCAC3 |
| 76 | OKJ | Okada Airlines Limited | Callsign: Okada Air, Name changed from Akada Air Service | WAN | AVCODES |
| 77 | OKJ | Akada Air Service | Name changed to Okada Airlines Limited | WAN | AVCODES |
| 78 | OKJ | Okada Airlines | | | ICAO1 |
| 79 | ÖKJ | Örebro - Köping Järnväg | 1897 - 1931 | S | svenska-lok.se |
| 80 | OKKT | Union Pacific R.R. Co. | added 4/1997; 7/2000-7/2005 | | NAKINA |
| 81 | OKKT | Missouri Pacific System | added 1/1990; 1/1994; eliminated 4/1997 | | NAKINA |
| 82 | OKKT | Oklahoma, Kansas and Texas R.R. Co. | added 10/1980; added 1/1981; eliminated 4/1982; added 1/1983; eliminated 1/1990 | | NAKINA |
| 83 | OKKT | Oklahoma, Kansas and Texas R.R. Co. | added 10/80; added 1/81; eliminated 4/82; added 1/83; eliminated 1/90 | | NRHS |
| 84 | OKKT | Missouri Pacific System | added 1/90; 1/94; eliminated 4/97 | | NRHS |
| 85 | OKKT | Union Pacific R.R. Co. | added 4/97; 7/2000 | | NRHS |
| 86 | OKKT | UP [MISSOURI PACIFIC] [OKLAHOMA KANSAS & TEXAS RR.] | | | PWRC |
| 87 | OKKT | Oklahoma, Kansas and Texas Railroad (Union Pacific Railroad) | | | RSRV |
| 88 | OKL | Oklahoma Department of Public Safety, Troop (O) | Callsign: Oklahoma, | USA | AVCODES |
| 89 | OKL | Oklahoma Department of Public Safety | | | ICAO1 |
| 90 | OKLA | OKLAHOMA BORDER EXPRESS INC | | | SCAC3 |
| 91 | OKLA | OAKLIFF TRANSPORTATION | | | SCAC4 |
| 92 | OKLB | OAKLEY, BRUCE INC | | | SCAC3 |
| 93 | OKLB | OAKLEY TRUCKING INC | | | SCAC4 |
| 94 | OKLD | OPTIKINETICS LTD | | | SCAC4 |
| 95 | OKLE | OAKLEY TRANSPORT INC | | | SCAC3 |
| 96 | OKLI | OKLAHOMA LIQUEFIED GAS COMPANY INC | | | SCAC3 |
| 97 | OKLI | O K LOGISTICS INC | | | SCAC4 |
| 98 | OKLR | OAKLEY RELOCATION | | | SCAC4 |
| 99 | OKLT | OAKLEY TRUCKING CO | | | SCAC4 |
| 100 | OKLX | Oklahoma Tank Car Line | 10/1966; eliminated 4/1969 | | NAKINA |
