“O” is for the Agfa Optima.
The Optima series of cameras by Agfa were among the first ‘automatic’ 35mm cameras. That is, by pressing the certain buttons, it would select the correct shutter speed and aperture for getting the correct exposures.
This was a coupled rangefinder camera. You still were responsible for focusing it.
Reference: McKeown’s Price Guide to Antique and Classic Cameras, 12th edition, ISBN 0-931838-40-1
The other cameras featured in this A to Z Challenge are linked on this page.