| 1 | SO | Sosoliso Airlines | ICAO: OSL, Callsign: Sosoliso, | WAN | AVCODES |
| 2 | SO | Swiss World Airways, S.A. | ICAO: SWO, Callsign: Swissworld, Ceased operations during 1998 | CH | AVCODES |
| 3 | SO | Southern Airways, Inc. | Callsign: Southern, | USA | AVCODES |
| 4 | SO | Salsa D'Haiti | ICAO: SLC, Ceased operations | RH | AVCODES |
| 5 | SO | Superior Aviation, Inc. | ICAO: HKA, Callsign: Spend Air, Designator SO recalled by IATA. IATA designator changed from AB* | USA | AVCODES |
| 6 | SO | Apex Airline Public Company Limited | Incorporated on 27th November 2012. Commenced operations in March 2015. | MYA | AVCODES |
| 7 | SO | Sun Air Co. | ICAO: SNR, Callsign: Sun Group, Designator S6 recalled by IATA. | SUD | AVCODES |
| 8 | SO | Syphax Airlines S.A. | Planned to start operations with 2 x CRJ-900. | TN | AVCODES |
| 9 | SO | South Orient R.R. Co., Ltd. | added 1/1993; 1/1994; eliminated 7/1994 | | NAKINA |
| 10 | SO | South Orient | added 1/1997; eliminated 7/1997; 7/2000 | | NAKINA |
| 11 | SO | South Orient R.R. Co., Ltd. | added 1/93; 1/94; eliminated 7/94 | | NRHS |
| 12 | SO | South Orient | added 1/97; eliminated 7/97; 7/2000 | | NRHS |
| 13 | SO | SOUTH ORIENT RAILROAD COMPANY, LTD. | | | PWRC |
| 14 | SO | SOUTH ORIENT RAILROAD COMPANY LTD | | | SCAC4 |
| 15 | SO | Serco Railtest | | UK | VKM |
| 16 | SOA | Skoda Air | Callsign: Skoda Air, | CZ | AVCODES |
| 17 | SOA | Southern Air Charters Co. Ltd. | IATA: PL, Callsign: South Aircharter, | BS | AVCODES |
| 18 | SOA | South Orange Avenue IBOA | | USA | TRAFFIC |
| 19 | SOAB | SOUTHSIDE FUEL | | | SCAC4 |
| 20 | SOAE | SOUND FREIGHT LINES INC | | | SCAC4 |
| 21 | SOAF | SOUTHERN OAKS LTD | | | SCAC4 |
| 22 | SOAG | SOTO AGUIRRE, LAURA | | | SCAC4 |
| 23 | SOAI | SOREIDOM | | | SCAC4 |
| 24 | SOAJ | SOUTHLAND TRANSPORT SERVICE OF STUT | | | SCAC4 |
| 25 | SOAN | SUBURBAN OIL COMPANY LLC | | | SCAC4 |
| 26 | SOAO | SHOOK, JASON | | | SCAC4 |
| 27 | SOAQ | SOLAR TRANSPORT COMPANY | | | SCAC4 |
| 28 | SOAS | SMITH OIL CO INC | | | SCAC4 |
| 29 | SOAV | SPORTSTATS | | | SCAC4 |
| 30 | SOAY | SANTOYO TRANSPORT | | | SCAC4 |
| 31 | SOB | Stabo Freight Limited | Callsign: Stabo, | Z | AVCODES |
| 32 | SOB | Stabo Freight | | | ICAO1 |
| 33 | SOB | Südostbahn | | CH | TRAFFIC |
| 34 | SOB | Schweizerische Südostbahn AG | | CH | VKM |
| 35 | SOBB | SONI | | | SCAC4 |
| 36 | SOBD | SCOOTIN & TOOTIN TRUCKING LLC | | | SCAC4 |
| 37 | SOBET | Sobetra, a.s. | | SK | VKM |
| 38 | SOBH | SOUTHERN BULK HAULERS INC | | | SCAC4 |
| 39 | SOBI | SOUTH BRITAIN OIL LLC | | | SCAC4 |
| 40 | SOBIE | Destylarnia Sobieski S.A. | | PL | VKM |
| 41 | SOBK | SOUND BIKES & KAYAKS INC | | | SCAC4 |
| 42 | SOBL | S O B MOTORS LLC | | | SCAC4 |
| 43 | SOBM | SIMMON FOODS INC | | | SCAC4 |
| 44 | SOBR | SORENTI BROTHERS INC | | | SCAC4 |
| 45 | SOBU | SOBOLAK INTERNATIONAL GMBH | | A | BIC |
| 46 | SOBU | ERES N. V. | | | SCAC3 |
| 47 | SOBU | SOBOLAK INTERNATIONAL GMBH | | | SCAC4 |
| 48 | SOBY | SOUTHEAST MILK INC | | | SCAC4 |
| 49 | SOC | SSA+, Joint-Stock Company | Callsign: Sochi, | RUS | AVCODES |
| 50 | SOC | Southern Cargo Air Lines | | | ICAO1 |
| 51 | SOCB | SOUTHERN CHARTERING PTY LTD (DIV OF | | | SCAC4 |
| 52 | SOCE | SOUTHERN CARTAGE INC | | | SCAC4 |
| 53 | SOCG | SOUTHEASTERN CARRIERS INC | | | SCAC4 |
| 54 | SOCH | Zakład Komunikacji Miejskiej Sochaczew | Real: ZKM, City: Sochaczew | PL | TRAFFIC |
| 55 | SOCHA | Przewóz Osób i Towarów Leszek Socha | | PL | TRAFFIC |
| 56 | SOCHI | MUP SochiAvtotrans | City: Sochi | RUS | TRAFFIC |
| 57 | SOCI | SOCIEDAD DE MOTORES DOMESTICOS S DE | | | SCAC4 |
| 58 | SOCK | SORCO PRODUCTS | | | SCAC4 |
| 59 | SOCN | SOUTHERN COUNTIES EXPRESS INC | | | SCAC4 |
| 60 | SOCO | SOCOMAC | | FR | VKM |
| 61 | SOCQ | SCOTT, ROBERT A | | | SCAC4 |
| 62 | SOCS | SOUTHEAST CONSOLIDATORS INC | | | SCAC4 |
| 63 | SOCT | STOC TRAILER SERVICES INC | | | SCAC4 |
| 64 | SOCU | STAR SERVICE SRL | | I | BIC |
| 65 | SOCU | STAR SERVICE | | | SCAC4 |
| 66 | SOCV | SOCATRA | | | SCAC4 |
| 67 | SOCX | Southern Oil Corp. | 8/1920 | | NAKINA |
| 68 | SOCX | Seatex Oil Co., Inc. | 1/1950 (not listed) | | NAKINA |
| 69 | SOCX | Sohio Oil Co. | 1/1994 (not listed) | | NAKINA |
| 70 | SOCX | BP Oil Co. | added 7/1995 | | NAKINA |
| 71 | SOCX | BP Amoco Chemical Co. | eliminated 1/2000 | | NAKINA |
| 72 | SOCX | BP Amoco Chemical Co., BP Oil Division | added 1/2000; 7/2000-7/2005; eliminated 10/2006 | | NAKINA |
| 73 | SOCX | BP Amoco Chemical Co., Chemical Division | added 10/2006 | | NAKINA |
| 74 | SOCX | Sohio Oil Co. | Jan-94 | | NRHS |
| 75 | SOCX | BP Oil Co. | added 7/95 | | NRHS |
| 76 | SOCX | BP Amoco Chemical Co. | eliminated 1/2000 | | NRHS |
| 77 | SOCX | BP Amoco Chemical Co., BP Oil Division | added 1/2000; 7/2000 | | NRHS |
| 78 | SOCX | *SOUTHERN OIL CORP. | | | PWRC |
| 79 | SOCX | *SEATEX OIL CO., INC. | | | PWRC |
| 80 | SOCX | +BP AMOCO CHEMICAL CO., CHEMICAL DIV. [BP OIL COMPANY; SOHIO OIL CO.] | | | PWRC |
| 81 | SOCX | BP Amoco Chemical BP Oil Division | | | RSRV |
| 82 | SOCX | BP AMOCO CHEMICAL COMPANY | | | SCAC4 |
| 83 | SOCY | SOUCY, RUSSELL S | | | SCAC4 |
| 84 | SOCZ | SOUTHERN COTTON OIL | | | PWRC |
| 85 | SOD | Aerolineas Sol, S.A. de C.V. | Callsign: Alsol, | MEX | AVCODES |
| 86 | SOD | Aerolineas Sol | | | ICAO1 |
| 87 | SODC | SOS LLC | | | SCAC4 |
| 88 | SODF | SOUTHERN TEXT WHSE | | | SCAC4 |
| 89 | SODG | SOCIETE D'ENTRETIEN GENERAL L A F I | | | SCAC4 |
| 90 | SODH | SOUTHERN COUNTIES TERMINALS | | | SCAC4 |
| 91 | SODI | SODIFER INC | | | SCAC4 |
| 92 | SODK | SONS OF DEROSE TRUCKING LLC | | | SCAC4 |
| 93 | SODL | SOURCE ONE TRANSPORTATION INC | | | SCAC4 |
| 94 | SODN | SIMMONS, DON SPORTS INC | | | SCAC4 |
| 95 | SODO | SODROX CHEMICALS LTD | | | SCAC4 |
| 96 | SODP | SOLDIER XPRESS LLC | | | SCAC4 |
| 97 | SODR | SCHROEDER MOVING SYSTEMS INC | | | SCAC4 |
| 98 | SODS | SODESA TRANSPORTACION S A DE C V | | | SCAC4 |
| 99 | SODU | UAB Sodų autobusai | City: Klaipeda | LT | TRAFFIC |
| 100 | SODV | SOUND DELIVERY SERVICE | | | SCAC4 |
