I recently purchased a 1986 Corvette. The digital dash worked fine until about a week ago when the bottom half of the tach went black. Shortly after an O/D icon appeared in tach area. Can anyone tell me if the tach light is just a bulb or is it time for repair and why the O/D icon appeared, neither the owners manualor service manual even show the icon on a 86

Remove dash,and check bulbs first, but these dashes are problematic too.
A lot of problems can be found in the connector, also the voltage supply regulator in the cluster can go bad too.

Remove dash,and check bulbs first, but these dashes are problematic too.
A lot of problems can be found in the connector, also the voltage supply regulator in the cluster can go bad too.
Can this also cause flickering?

Can this also cause flickering?
I believe that would be a safe assumption.

Can this also cause flickering?

Yes, its the voltage regulator in the dash.
I just had mine overhauled by a company in Virginia, it was $285 all included, it's not cheap but it is very difficult to repair these yourself.