| Abbreviation | Definition |
|---|---|
| OOM | mmO2, PLC American Depositary Receipts |
| OPJ | Ohio Power Company |
| OQT | Quaker Oats Company |
| ORB | Orbital Sciences Corporation |
| ORH | Odyssey Real Estate Holdings Corporation |
| ORI | Old Republic International Corporation |
| OS | Oregon Steel Mills, Inc. |
| OS | Oregon Steel Mills, Incorporated (de-listed) |
| OS | Oregon Steel Mills, Incorporated (de-listed) |
| OSG | Overseas Shipholding Group I |
| OSK | Oshkosh Truck Corporation |
| OSM | Osmonics, Incorporated |
| OST | Austria Fund, Inc. |
| OTE | Hellenic Telecommunications |
| OTL | Octel Corporation |
| OUI | Realty Income Corporation |
| OV | One VY Bancorp, Inc. |
| OV | One VY Bancorp, Incorporated (de-listed) |
| OV | One VY Bancorp, Incorporated (de-listed) |
| OWC | Owens Corning |