A floating marine facility, such as a buoy, plays an important role as an aid to navigation for maritime safety. Due to development of the wave energy conversion system, installing an oscillating water column (OWC) with rechargeable batteries on a buoy has become the trend. In order to extend the battery replacement cycle, the battery voltage must be kept within a certain range by improving the energy conversion efficiency. This paper suggests using inlet geometry modification of the OWC to increase vertical displacement. Two types of inlet geometry, the trumpet type and the cylindrical type, are designed and compared with each other through a scale model experiment in a 2-D wave tank, as well as through a full-scale experiment at sea. The results show that the trumpet-shaped inlet of the OWC generates more electric power than does the cylindrical-shaped inlet.