Hello joshrm115, my name is Josh as well.
To answer your question. No offense man, but do a little research on the fish that you have. Your oscar is gonna fill that 100g tank within a year. It may not seem like it, but give that fish a couple of months and you will notice the difference.
Do you have any pics of this tank? I just wanna see all of these fish in the same tank to see what size they are.
There's a chance you'll have problems with that dempsey down the road, too. I've seen a 7" #### male drag a common pleco by its tail and bash it to death. It was not pretty. If it's a juvie, you might be fine for a while.
I don't know a whole lot about knife fish so can't help you there. I just know they get pretty big. I would imagine as long as you have places for it as well as the other fish to hide, the knife might be fine. Correct me if I'm wrong.
Firemouths are awesome fish. ( Well, all of your stock is pretty cool

) I got a male FM about 8 months ago and don't regret it. Im sure your firemouth can throw down with the oscar if aggression appears. You could add a female to the mix. Watch out though when they decide to breed. Your could have some nasty fights between the pair and that dempsey.
You definitely have the room for all of the fish you have. I just think you don't have room for all of them together. Just my opinion, so take it as you want to.
I think you should consider a couple of different options for those fish.
Either keep it as an Oscar only tank and let that fish have the ENTIRE tank to himself.
Go for a species tank with the firemouths and you could probably get away with a couple of pairs of them.
If your gonna keep the JD, beef up the tank with hiding places for the other fish, as I stated earlier.
Or just disregard my suggestions and do whatever you want, man.
Regardless to what advice you're given, it's your tank so really its up to you if want another fish or not. Just don't add another fish just because you're bored of the ones you have. I've made that mistake a few times and it never worked out so far.
Good luck fellow Josh,
BobRoss