Adjustable Objective
Also: AO
A parallax adjustment built into the scope's objective bell instead of a side-focus turret. The front ring rotates to match the target distance.
AO scopes are a common older layout. The front bell is marked with yardage values (50, 100, 200…) and rotating it focuses the parallax plane to that distance. Effective but slow if you’re switching ranges quickly.
Most modern precision scopes have moved to side-focus turrets for the same function, which is faster to operate without breaking position.