Abstract: Oscillating-buoy is a major type of wave energy converters (WECs) that convert wave energy into hydraulic or electric energy, and its performance significantly affects its availability and efficiency of wave energy utilization. This paper presents an experimental study on the operation characteristics of oscillating-buoy wave energy converters, and analyzes the effect of nonlinear external load, geometric arrangement and mooring system on dynamic responses, flow rate and conversion efficiency. The results show that for a fixed size WEC, the maximum efficiency can be achieved at its optimal wave frequency and external load. The optimal geometric arrangement depends on external load. For large external loads, a single buoy WEC has an efficiency greater than other geometric arrangements. The efficiency of a two-buoy WEC is increased when fewer mooring constraints are imposed.