The O/D light flashing is a VERY bad sign, it could be any number of things, as they have several well known faults.
It will NOT illuminate however due to low fluid, you will just lose drive, is it jumping gears yet, has it locked in thrid at all(refused to change from third), have you lost lock up in
3rd or 4th.
One of the most common things is the turbine speed sensor, located on the "drum" looking part accessible behind nearside wheel with a two pin connector going to it, about £20 but a bitch to change if stuck!!
Another is the valve selector as it is fully electronic, like ferraris etc are(tip tronic) automatics!!, these are around £35.
Most of the box is rebuildable without taking out of the car, but if it has a damaged clutch basket for example it will need a full rebuild.
Sniff the fluid which should be cherry red in colour, if dark or smells burnt get your money back, or shell out £600-£800 soon for a new box.
Taste the fluid, if it is acid tasting give it back as well.
My best advice would be go to your nearest decent autobox repairers and get a free evaluation, where they will drive it and tell you what is what, as it is a lot easier than by desciptions.
One major problem is these boxes desperately need even if standard a far better oil cooler, the boxes overheat all the time, tha US cars that use these boxes all have ducted air cooling around the gearbox.
tabetha