When I had my oscar, he never had full blown fin/tail rot, however he would scratch himself on gravel/rocks quite offen. If I did not see him do this, they would start to get infected. White fungus would begin to grow on the wound. I would dose with melafix for three days, and it always took care of the problem. Normaly after the first 24 hours the visible signs of the fungus were gone.
However the rot seems like it is being rather aggresive if you think its already spreading to the dorsal and anal fins, so you may want to just get a stonger med as the others have mentioned.