Definition
A 75 MHz radio beacon that marks a fixed point on the final approach course of an Instrument Landing System (ILS), typically located 4 to 7 miles from the runway threshold. As the aircraft passes over the OM, the airborne marker beacon receiver triggers a blue light and a continuous series of low-tone dashes (about 400 Hz), confirming the position where the glideslope intercept normally occurs at the published altitude.
Plain English
A radio signal on the ground that tells you the moment your aircraft passes over a specific point on the approach path to the runway. When you fly over it, a blue light flashes in the cockpit and you hear a steady series of low-pitched beeps.
Context Anchor
Seen on some instrument approach charts, approach diagrams, and cockpit indications during an instrument approach to a runway.
Derivation
Called "outer" because it sits farthest from the runway among the marker beacons on an ILS approach. The middle marker (closer in) and inner marker (closest) follow the same naming logic. "Marker" simply means it marks a specific point along the approach.
Why Pilots Care
It gives pilots a precise position fix on the approach so they can confirm they are on the correct glide path and decide whether to continue or execute a missed approach.
Analogy
It is like a mile marker on a road: it does not steer you, but it confirms exactly where you are along the route.
Intuition Check
Do not read outer as meaning outside the airport or outside controlled airspace. Here, outer means farther from the runway than marker points closer to the runway.
Example Sentence 1
Crossing the OM at 1,800 feet, the pilot confirmed the glideslope was alive and continued the approach.
Example Sentence 2
Crossing the OM, the aircraft was still in solid IMC but aligned with the localizer.