O-2 vs O-2A vs O-2B
 

I'm trying to find out a little more about the differences between the models...one feature I've noticed is the extension of the pilots window up and towards the skylights. Some planes labeled "O-2A" have this feature, some don't. It seems all the ones I've seen labeled "O-2" don't have it - which makes sense, as those were the early ones.

Does anyone have more info on when this change actually showed up?

"After delivery of the initial order of O-2As (Ser# 67-21295 thru 67-21439), the USAF asked Cessna to make some modifications as a result of experience in the field. Most important of these was enlarging the window on the pilot's side for better visibility in turns to the left. The window also bulged out a bit, improving downward visibility out the left side. This change included an external emergency window release handle, allowing rescue crews to get immediate access to the pilot. According to pilots who have flown both airplanes, the bulging window is supposed to reduce the speed of the O-2A by as much as four knots. This reported decrease in performance may be airplane specific, rather than valid across the modification. The USAF Dash 1 does not indicate any change in performance for early vs. late models and no two O-2A's seem to have the same performance, anyway. (In talking to other O-2A owners, mine seems a few knots faster than others.)

The new window design required replumbing the fuel lines, so that the fuel selector switch for the left tank (feeding the front engine) was moved to the right side of the cockpit. You might think that having both the left and right fuel selectors on the right side of the cockpit could create some confusion, but it actually solved a different problem. The left fuel tank selector was now to the left, or forward of the right fuel tank selector. This now made it clear which engine was drawing fuel from that tank. The second delivery of O-2A's (68-6857 thru 68-6903) had the same layout of the earlier 145 planes, but all deliveries from 68-10828 and higher are with the larger, bubble window."

Taken from http://www.o2cricket.com/the-o-2a/

Thank you!