OD light flashing simply indicates that the engine management system has identified a transmission malunction. This could be the result of a multitude of problems. We’re going to need some additional info from you. Having the actual faults codes listed here would give us a starting point.
You stated that the transmission shifts roughly, curious does it shift roughly in all gears (1st thru 4th). How about park to drive range, park to reverse, reverse to drive?
Are all gears present (1st thru 4th)? Are all upshift ans downshifts harsh?
You concern may not be a mechanical issue within the transmissions but rather an electrical circuit fault.