Observability – Occupations in electrical/electronics engineering – Ohm – Ohmmeter – Ohm's law – Oil shale – One-line diagram – On-premises wiring – Open-circuit test – Open circuit voltage – Open-circuit time constant method – Open-circuit voltage – Operational amplifier – Optical fiber – Optimal control – Oscillation – Oscilloscope – Oudin coil – Out of phase – Outline of electrical engineering – Overhead line – Oversampling – Overshoot (signal) – Overvoltage – Oxidation – Oxygen – Oxygen-free copper –