Definition
The omni bearing selector is the rotatable course-selection knob and associated azimuth ring on a VOR indicator. The pilot turns the OBS to choose a specific magnetic course (radial) to or from a VOR station. The selected course appears at the top index of the compass card, and the course deviation indicator (CDI) then shows the aircraft's position relative to that chosen course.
Plain English
It's the knob you turn on a VOR instrument to pick which course line you want to fly to or from the station. Once you've picked the course, the needle on the instrument tells you whether you're left or right of it.
Context Anchor
You will use or see the OBS during instrument navigation, including arrivals and approaches, when selecting a course to track toward or away from a navigation point.
Derivation
Omni' comes from Latin 'omnis' meaning 'all' — a VOR transmits bearings in all directions (360 radials). 'Bearing' is the direction to or from the station. 'Selector' is the knob that lets the pilot select which of those 360 bearings to use. So: the knob that selects one bearing out of all available bearings.
Why Pilots Care
It lets the pilot establish and maintain a precise track, which is essential for staying on the published arrival route in instrument conditions.
Intuition Check
The OBS does not tune the navigation station. It selects the course line the instrument will display after the navigation source is already chosen.
Example Sentence 1
On arrival, the pilot set the OBS to 045 to track inbound to the VOR on the 225 radial.
Example Sentence 2
With the OBS set correctly the needle centered, confirming the aircraft was flying the selected path to the station.