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 degrees/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. (C) 2018 Elsevier Ltd. All rights reserved.