First of all, you need a new mechanic.
The flashing O/D light indicates an electronic fault in the transmission controls. If you read your Owner’s Manual, it should confirm this. When an electronic fault in the transmission’s controls is not fixed promptly, it will result in MUCH more expensive mechanical repairs to the transmission.
While the car very likely did need to have the transmission fluid changed, that is sort of like rearranging the deck chairs on the Titanic at this point, i.e.–it is a good idea, but it is not going to resolve the electronic fault indicated by the presence of the flashing O/D light.
The fact that the tachometer shows the revs going up and down while driving indicates that slippage in taking place in the transmission, so damage may already taken place, but I would suggest that you get this car to an independent transmission shop (NOT AAMCO, Lee Myles or Cottman) for an assessment a.s.a.p.
As to why the CEL is still lit up, it could be that you have not driven the car sufficiently for the trouble codes to be cleared from the computer’s memory. Or, it could mean that your mechanic did not resolve all of the problems with this car.