Plain English Meaning

The OBS is the knob you turn on a VOR instrument to pick which line out from the station you want to fly. Once you've picked it, the needle on the instrument tells you whether you're on that line or off to one side.

Full Definition

The Omnibearing Selector (OBS) is the rotatable control knob on a VOR indicator used to select the desired course (radial) to or from a VOR ground station. Turning the OBS rotates the compass-like azimuth ring on the instrument face, choosing which of the 360 possible radials the pilot wants to fly along, while the course deviation indicator (CDI) shows whether the is or of that selected .