An Optical Particle Counter (OPC) has been designed with a single ellipsoid reflector as the main optical element. Photodiodes with low noise transimpedance are used to detect forward and backscatter responses of particles scattering laser light (685 nm) at the ellipsoid focus. The Zemax (R) ray tracing program has been used to estimate the OPC's light collection efficiency. The design process has included modeling the optical response of various lenses, reflector coatings, and laser wavelengths. Simulations show that our OPC with a single ellipsoid reflector has 58.2% total efficiency for collecting forward and backscattered light.