Yes, you can pull the codes for the flashing overdrive light if you pull them before turning the car off and after the light starts to flash. If you have a code reader, then just drive around until the light comes on, pull over and hook up your code reader before turning the car off. If your reader only works in KOEO conditions, then you will not be able to retrieve the codes.
KOEO - Key On, Engine Off
The reason it is shifting harshly, after the light starts flashing, is because the PCM has commanded the tranny be run in a Fail Safe Mode. In this mode, the trannys line pressure is up'ed to higer than normal pressure under all conditions. This higher than normal line pressure is what is causing the harsh shifts.
Get those codes and let us know what they are. We will see if we can figure out what is going on from there.
