Owner at a now out of business LFS told me that Hexamita can be brought upon by dirty water and nutritional deficiencies. He actually told me to remove the carbon out of my filter (I never use carbon other than to remove meds). Whatever grain of truth lies in here, I really don't know. Oscars are Omnivores, so try adding in some veggies. I have recently started feeding my cichlids dried seaweed. Not the kind they use for Sushi rolls, but this matted stuff. My marines don't like nori, but they gobble up that other stuff. My cichlids seem to really like it too. Bonus is that it is about 1/3 the price of Nori - not that it is expensive to begin with.
My suggestion: If you have followed the above advice, the HITH should be gone. Those craters will still be there until it heals over, but you will still see 'indents'. When your fish get a bad case of it, it will usually never heal over to where it started. I recommend NOT feeding Beefheart - gets the water way too dirty. The only meat I would feed it is whole frozen fish, shrimp that they will mostly swallow, and the occasional piece of liver (high in vitamins). Cut the liver way smaller than bite size, as you don't want your oscar to keep chewing it and having a big cloud dissolve in your tank. Add in some seaweed, spinach or any type of vegetable. Keep trying various veggies until your Oscar starts accepting some. Don't leave veggies in the tank for too long and add to the bio-load of your tank. Get a bigger tank when you can. 25% water changes/week. Clean your filter media the day before you do a water change in some aquarium water. You need all the bacteria on the media as you can keep, or you will make matters worse.