yep, orange flashing o/d light is internal gearbox prob. There is a box in the engine bay which when shorted out can give the error code (number of blinks of the light).
I had mine checked and tested by an auto box specialist and there were no internal major problems. The box was changing properly and holding pressure fine through all the gears. The problem with the light was a lazy solenoid in the box. The solenoids are meant to fire in within a certain time and if they are lazy as mine was, it sends the error, but i'm not saying this is your problem, as the flashing o/d covers many faults/errors.There are, I believe 4 different speed solenoids in the box.
The solution to fix mine was rather extreme...................my missus smashed it into a tree
,wrote it off (not on purpose, I might add) and we bought another one..................with no flashing light
Have you also checked your tranny fluid level???
hope you get it sorted, I'll try and find some more info for ya