any natural or manmade obstruction that canalizes, delays, restricts, or diverts movement of a force. The effectiveness of an obstacle is enhanced considerably when covered by fire. Obstacles can include abatis, antitank ditches, blown bridges, built-up areas, minefields, rivers, road craters, terrain, and wire. Obstacles are classified as either existing or reinforcing. OCOKA observation and fields of fire, cover and concealment, obstacles, key terrain, avenues of approach OD other detainee ODCSPER Office of the Deputy Chief of Staff for Personnel OF observed fire ofcr officer OIC officer in charge OP observation post OPCON operational control