| Word | Description |
| Occupancy | The proportion of time a detector is occupied. Occupancy is a pseudo measure of density on a roadway. |
| OFE | Other Freeways & Expressways |
| OHM | The unit of electrical resistance equal to the resistance though which a current of one ampere will flow when there is a potential difference of 1 volt across it. |
| Omega | Omega Classifier |
| OPA | Other Principal Arterial |