A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Obstacle clearance altitude/height - OCA/H - The lowest altitude or the lowest height above the elevation of the relevant runway threshold or theaerodrome elevation as applicable, used in establishing compliance with appropriate obstacle clearance criteria

Outer marker - OM - A marker beacon at or near the glideslope intercept altitude of an ILS approach. It is keyed to transmit two dashes per second on a 400 Hz tone, which is received aurally and visually by compatible airborne equipment. The OM is normally located fou to seven miles from the runway threshold on the extended centerline of the runway.