A flashing O/D light is a warning of a Transmission problem.
Depending upon how long the condition has been going on, you may be looking at a $400 fix to replace the Transmission Valve Body and/or shift solenoids to a very expensive transmission rebuild or replacement. Typically, the problems is caused by a leaking valve body gasket, or a sticking valve body solenoid. This cause the electronic shift solenoids to be overworked and become weak. When the shift solenoids are weak the cause slipping which causes excessive heat and damages the friction clutches and bands. If you catch it early enough, you can often get by with a simple Valve Body, Solenoid repair. If you don't catch it soon enough, it can trash the transmission.
I know this is not the news you wanted to hear, but better to know now before you get hit with a huge repair bill.
...Rich