First of all, welcome to the site.
Let's try and answer some of your questions. Hole in the head disease can result in severe scarring, very much like very bad acne. If it's a very mild, it should heal up okay but you still may see marks where it was.
Good-quality food does contain vitamins. However, it is a good idea to supplement this with live food if and when possible. Try and refrain from giving them shop bought live fish (often referred to as feeder fish) many pet shops sell mealworms and crickets, these are absolutely brilliant for Oscars. You can also feed them on worms and shrimp. If you don't like lively, frozen food is also good, freeze-dried even better. Have a look through the freezer at your local fish store, they often sell a large amounts of frozen food which will be perfect for your Oscar.
Basically, venting a fish means examining its undersize to determine the sex. A male and a female Oscar have two holes, both of much will be slightly different on each fish. You will have to remove the fish from the tank and probably use a magnifying glass to the absolutely sure. If you see that the two Oscars vents are different, the chances are you have a male and a female. All you have to do then is wait for them to start breeding, you'll then know which one is the female. If the two events are the same, you could either have two females, or to males. Personally, I think it would be best to wait and see what happens, your Oscars are 9 inches, I would say there are probably sexily mature, if not, they are quite close. It's nice when fish do start breeding, it does sort of proof that they are happy in their environment and you are looking after them properly. However, there is a downside, they can often produce many offspring which may survive, you then have to do think about what you are going to do with them.
There is a cliche that has gone around for a long long time, "fish grow to the size of their tank" I suppose in some cases, you could say yes, they do. However, in these circumstances, they are often stunted because of poor water conditions because of their environment. A healthy Oscar should reach at least 10 inches. From my experience, they should be at least seven to 8 inches within the first year. You got to remember that fish are often two or three months old. My Oscars are probably approaching 8 inches, they are around about a year old now. An Oscar will probably take a couple of years to reach 12 inches, sometimes not quite as long.
It does sometimes depend on what you feed Oscars to what their faeces like and what colour it is. Red/Yellow/white faeces can often be a sign of parasites. I'm not saying this is your problem, just keep an eye on it, post some photos if you can.
Hope this helps you. I am sure other people will give you the benefit of their knowledge as well
