Ocean wave energy conversion as one of the renewable clean energy sources is attracting the research interests of many people. This review introduces different types of power take-off (PTO) technology of wave energy converters. The novelty of this paper is to present advantages and disadvantages of the linear direct and indirect drive PTO devices for ocean wave energy conversion. The designs and optimizations of PTO systems of ocean wave energy converters have been studied from reviewing the recently published literature. The novel mechanical designs of the PTO systems have been compared and investigated in order to increase the energy harvesting efficiency.