The biggest single fish.I was thinking of getting an Oscar,but after some research I now know that I can't.Is there any other large fish that is similar that I could keep?
Not sure, but one Oscar could be okay though it may be a bit lonely. Better to go with some smaller fish in groups that get along, like live bearers and tetras. 180 liters is a good size for a good community tank. Bristle-nose cats, cories, tetras, Swordtails, Guppies, Danios so on and so forth. Some loaches may even be a good bet.
In a 45 gallon tank... Have you considered mbuna cichlids? Go check out the African Cichlids section. It's not just big fish that are interesting or have character! Seeing as you were interested in an oscar, I figure a different cichlid or cichlids would be your thing but it's best to stick with the more manageable sizes.