| 1 | UO | Helicopters Hong Kong Limited | ICAO: HHK, Callsign: Helihongkong, Name changed to Hong Kong Express Airways Limited | HK | AVCODES |
| 2 | UO | Trans-Union | | F | AVCODES |
| 3 | UO | Empreas Aero Uruguay, S.A. | ICAO: AEU, Callsign: Uniform Oscar, | ROU | AVCODES |
| 4 | UO | Helicopters Hong Kong Limited | ICAO: HHK, Callsign: Helihongkong, IATA designator changed to UO. Name changed from Helicopters Hong Kong Limited | HK | AVCODES |
| 5 | UO | Hong Kong Express Airways Ltd | ICAO: HKE, Callsign: Hongkong Shuttle, | HK | AVCODES |
| 6 | UO | Union R.R. of Oregon | 1/1975; added 7/1979; 1/1980; eliminated 4/1986; eliminated 4/1989 | | NAKINA |
| 7 | UO | Union R.R. of Oregon | 1/75; added 7/79; 1/80; eliminated 4/86; eliminated 4/89 | | NRHS |
| 8 | UO | RETIRED 08-23-93 ATP (000) [UNION RAILROAD OF OREGON] | | | PWRC |
| 9 | UO | Union Railroad of Oregon | | | RSRV |
| 10 | UO | UNION RAILROAD OF OREGON | | | SCAC4 |
| 11 | UO | Union Railroad of Oregon | | USA | TRAFFIC |
| 12 | UOAA | UNIVERSITY OF ALBERTA | | | SCAC4 |
| 13 | UOAF | UNION OIL AND FARMERS UNION | | | SCAC4 |
| 14 | UOBC | UNIVERSITY OF BRITISH COLUMBIA, THE | | | SCAC4 |
| 15 | UOBT | UNITED SEA CARGO LLC | | | SCAC4 |
| 16 | UOCR | UNITED OIL OF THE CAROLINAS INC | | | SCAC4 |
| 17 | UOCS | UNIVERSITY OF CALIFORNIA SAN DIEGO | | | SCAC4 |
| 18 | UOCX | Union Oil Co. of California | 8/1920-1/1952 | | NAKINA |
| 19 | UOCX | General American Transportation Corp. | 7/1954-7/1965 | | NAKINA |
| 20 | UOCX | Union Oil Co. of California (General American Transportation Corp.) | 10/1966 | | NAKINA |
| 21 | UOCX | General American Transportation Corp. | 4/1970-1/1994; eliminated 1/1998 | | NAKINA |
| 22 | UOCX | General American Marks Co. | added 1/1998; 7/2000-7/2005 | | NAKINA |
| 23 | UOCX | Union Oil Co. of California | 4/38-1/52 | | NRHS |
| 24 | UOCX | General American Transportation Corp. | 10/57-7/65 | | NRHS |
| 25 | UOCX | Union Oil Co. of California (General American Transportation Corp.) | Oct-66 | | NRHS |
| 26 | UOCX | General American Transportation Corp. | 4/70-1/94; eliminated 1/98 | | NRHS |
| 27 | UOCX | General American Marks Co. | added 1/98; 7/2000 | | NRHS |
| 28 | UOCX | GENERAL AMERICAN MARKS CO. [(GATC) GENERAL AMERICAN TRANSPORTATION CORP] UNION OIL CO. of CALIF.) | | | PWRC |
| 29 | UOCX | Union Oil Co. of California (GATX) | | | RSRV |
| 30 | UOEX | Union Oil Co. of California | added 10/1984; eliminated 1/1987 | | NAKINA |
| 31 | UOEX | UNOCAL | added 1/1987; eliminated 7/1992 | | NAKINA |
| 32 | UOEX | UNO-VEN | added 7/1992; 1/1994; eliminated 4/1998; 7/2000-7/2005 (not listed) | | NAKINA |
| 33 | UOEX | Union Oil Co. of California | added 10/84; eliminated 1/87 | | NRHS |
| 34 | UOEX | UNOCAL | added 1/87; eliminated 7/92 | | NRHS |
| 35 | UOEX | UNO-VEN | added 7/92; 1/94; eliminated 4/98; 7/2000 | | NRHS |
| 36 | UOEX | UNO-VEN [UNOCAL; UNION OIL CO. OF CALIFORNIA] | | | PWRC |
| 37 | UOEX | UNO-VEN | | | RSRV |
| 38 | UOEX | UNO-VEN | | | SCAC4 |
| 39 | UOGA | UNITED OIL GRE | | | SCAC4 |
| 40 | UOGX | United Oil & Natural Gas Products Corp. | 7/1923 | | NAKINA |
| 41 | UOGX | UNITED OIL & NATURAL GAS PRODUCTS CORP. | | | PWRC |
| 42 | UOGZ | UNIVERSITY OF GEORGIA | | | PWRC |
| 43 | UOIC | UNNERSTALL OIL COMPANY | | | SCAC4 |
| 44 | UOIG | URBAN ORGANICS INC | | | SCAC4 |
| 45 | UOIK | UNITED OILFIELD INC | | | SCAC4 |
| 46 | UOIN | UNIT ONE INC | | | SCAC4 |
| 47 | UOKX | Union Oil Co. of Canada, Ltd. (Union Oil Co. of California) | 7/1930-7/1935 | | NAKINA |
| 48 | UOKX | Union Oil Co. of Canada, Ltd. | 7/1937-1/1945 | | NAKINA |
| 49 | UOKX | British American Oil Co. Ltd. | 1/1946-4/1947 | | NAKINA |
| 50 | UOKX | Union Oil Co. of Canada Ltd. (Union Oil Co. of California) | Apr-38 | | NRHS |
| 51 | UOKX | BRITISH AMERICAN OIL CO. [UNION OIL CO.] | | | PWRC |
| 52 | UOLC | UNITED OIL CO | | | SCAC4 |
| 53 | UOLT | UNITED OIL TRANSPORTATION | | | SCAC4 |
| 54 | UOMN | UNISHIPPERS OF MICHIGAN | | | SCAC4 |
| 55 | UONA | ULTIMATE ONE (1) | | | SCAC4 |
| 56 | UONE | USA1 LOGISTICS INC | | | SCAC4 |
| 57 | UOPX | Union Oil Co. of Pennsylvania | 7/1923 | | NAKINA |
| 58 | UOPX | Union Oil Co. of Pennsylvania, Inc. | 7/1937 (not listed) | | NAKINA |
| 59 | UOPX | UOP, A New York Partnership | added 1/1997; eliminated 7/1997; 7/2000-7/2005 (not listed) | | NAKINA |
| 60 | UOPX | UOP, A New York Partnership | added 1/97; eliminated 7/97; 7/2000 | | NRHS |
| 61 | UOPX | UOP A NEW YORK PARTNERSHIP [UNION OIL CO.] | | | PWRC |
| 62 | UOPX | UOP New York | | | RSRV |
| 63 | UOPX | UOP A NEW YORK PARTNERSHIP | | | SCAC4 |
| 64 | UOSM | UNITED OCEAN SHIP MANAGEMENT PTE LT | | | SCAC4 |
| 65 | UOT | UNICOM OIL TERMINAL SA | | RO | VKM |
| 66 | UOTC | UNITED OVERSIZED TRUCKING INC | | | SCAC4 |
| 67 | UOTI | UNID OCEAN TRANSPORT INC | | | SCAC4 |
| 68 | UOTP | UPLAND ONE (1) TRANSPORTATION | | | SCAC4 |
| 69 | UOUI | UNION OIL OF UNION IL INC | | | SCAC4 |
