A novel technique to measure the frequency-dependent hydrodynamic parameters of an oscillating water column (OWC) using tank testing is proposed. This technique arose from investigations into the effect on the power absorbed by OWCs of air compressibility in the chamber above the water column. Two models of an OWC were constructed for that investigation. For the first model, the volume of the air chamber above the water column was scaled geometrically by the factor to which the water column was scaled. For the second, the volume of the air chamber was scaled by the square of the factor to which the water column was scaled. The proposed technique to measure the hydrodynamic parameters,which eliminates the need for a forced-oscillation rig, uses these two OWC models, and relies on water column motion which takes place due to air compression when the air chamber of the second model is sealed from atmosphere. The technique described for monochromatic waves. Results obtained from the testing of an OWC are presented. Spectral methods are applied to the technique, and then implemented to measure the parameters of the model OWC. This allows multiple frequencies to be analysed using a single set of tests.