| Ohm: | Unit of resistance; 1 ohm = 1volt/ampere. |
| Ohm's law: | The current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor. |
| Open system: | A system across whose boundaries both matter and energy can pass. |
| Optical fiber: | A long, thin thread of fused silica, used to transmit light, based on total internal reflection. |
| Oscillatory motion: | The to and fro motion of a body about its mean position. |
|