| Abbreviation | Definition |
|---|---|
| ONW | Oregon and Northwestern Railroad |
| ONYX | American Colloid |
| OOCZ | Orient Overseas Line |
| OOCZ | Orient Overseas Line Incorporated |
| OOLU | Orient Overseas Container Line |
| OOLU | Orient Overseas Container Line Incorporated |
| OOOX | Osborne |
| OOOX | Osborne Incorporated |
| OOUU | Orient Overseas Line |
| OOUU | Orient Overseas Line Incorporated |
| OPE | Oregon Pacific and Eastern Railway Company |
| OPE | Oregon, Pacific and Eastern Railway |
| OPIX | Equistar Chemicals |
| OPIX | Equistar Chemicals LP |
| OPPX | Omaha Public Power District |
| OPR | Ontario Pacific Railway |
| OPR | Oregon Pacific Railroad |
| OPR | Oregon Pacific Railroad Company |
| OPR | Ottawa & Prescott Railway |
| OPSL | Open and Prepay Station List |