The use of conventional capacitance-based deep-level transient spectroscopy is not applicable when defect concentrations approach the background carrier concentration. Due to this limitation the technique cannot be used for examining heavily irradiated silicon, or semi-insulating semiconductor materials. Optical deep-level transient conductance spectroscopy can overcome the limitations of capacitance-based techniques through the measurement of a conductance transient measured with a marginal oscillator. This paper provides details of the application of this method to heavily damaged high-purity silicon. Silicon-based PIN detector structures irradiated with 1 MeV neutrons, to approximately 3×1013 n/cm2 and detectors irradiated with 24 GeV/c protons, to 3.8×103 p/cm2, were examined.