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 aircraft is left or right of that selected course.
Plain English
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.
Context Anchor
Seen on VOR indicators and course deviation indicators when setting or reading a navigation course.
Derivation
Short for Omnibearing Selector. 'Omni' comes from Latin meaning 'all', and 'bearing' is the direction from one point to another. So an omnibearing selector is the control that lets you select any one of all possible bearings (360 of them) from the VOR station.
Why Pilots Care
Precise radial selection enables accurate course tracking, intercept procedures, and instrument approaches.
Intuition Check
Do not think of the OBS as the whole navigation instrument. The OBS is the control used to set the course the instrument will display.
Example Sentence 1
After tuning the VOR frequency, the pilot rotated the OBS to 270 to track inbound on the selected radial.
Example Sentence 2
With the OBS centered, the CDI showed the aircraft was on the selected course.