Yeah I'd say you're maxed out already on your bioload. Oscars are incredibly messy. I would say in the longterm you'll need to choose between your Oscar and your Green Terror because, if the Terror is a male, it will likely reach 10" or more, and males can pick fights in my experience. It might work with all of your fish but, as oscarsftw said, you'll need to do really large water changes to keep your nitrate level down, as in 50% water change at least weekly. Also, Oscars are prone to getting a horrible disease called "Hole in the Head" that is brought on by bad water quality. Let me rephrase that: Oscars need pristine water quality to avoid getting this disease. So you'd be running the risk of that occurring as well...
What kind of filtration do you have on your tank right now? With that many fish, you'll want to aim for hourly filtration of ten times your tank volume. In other words, you'll want your total filtration to be 900 gallons per hour. This can be achieved with a good canister filter and one or two HOB (hang-on-back) filters.
You mentioned you're new to the fish hobby so I'll just throw this in: Keeping Central American (CA) and South American (SA) cichlids is a really fun and rewarding experience but you can't stock your tanks like you can with African cichlids. All of the species you've got like to establish their own territories and often won't tolerate others in that area. Your Dempsey, Terror, and Oscar will all need sizeable territories and will fight if someone rubs them the wrong way.
