Ack! A rotten egg smell?? That means hydrogen sulfide; VERY deadly and a result of anaerobic bacteria at work. Get ALL the fish out of the tank, and do an almost complete water change. Prior to that, do a major gravel vac as the anaerobic pocket may be in there. After the water change, change the filter completely: thats the other place it may be occurring. Anaerobic bacteria shows up in areas where
O2 can't reach; deep sand beds that
O2 can't get into, uncleaned/changed filters that have become completely clogged, areas where the gravel is covered by decor and keeping the
O2 from reaching it, or decorations with places where water can't circulate. Do check for any of those things, and make changes if needed. Your tank may recycle; keep an eye on your water parameters and water change (maybe 20-30%) anytime ammonia or nitrites show up. In your case, you don't want ANY in your tank as you don't want to stress your fish at all.
To be honest, I'm not sure if the hydrogen sulfide is the cause of the popeye, scabs, etc tho; it may be the stress from living with the hydrogen sulfide that has reduced the oscar's immune systems and made them susceptible to disease.
Start with the major water changes and get the poisons out of the tank. That MAY be enough to kick start some healing. In the mean time, can you give us a better description of the scabs or get a pic? Any changes in behavior? Heavy breathing noted? I don't want to recommend meds in case its (for example) a parasite and I recommend antibiotics which wouldn't help.