The fundamental hydrodynamic responses for four different oscillating wave surge converters (OWSCs) variants are analysed and compared. The four variants include; an isolated flap, a flap in front of a wall, a flap inside a caisson with an isolated water chamber behind the flap and a flap inside a caisson with a water chamber behind the flap that is linked to the water outside via to a gap under the flap. The flap rotations and power captures for the four variants are obtained using a frequency-domain model, with the flap response amplitude operator constrained to be less than 15°/m and the power capture maximised for damping-only control. It is shown that the four variants have qualitatively different frequency-dependencies in rotations and power captures, which are shown to be linked to differing values for the incident wave torque and system natural frequencies. A notable discovery was the existence of a potentially-exploitable water chamber pumping mode for a flap in a caisson with a gap under the flap, which had not previously been identified. The (dis)similarities between the flap variants are identified and discussed, providing a method by which the relative performance of the different variants can be understood.