Personally think a 150 gallon or bigger is about the right size for a pair. The 110 ish tank would be pushing it. You might be able to get away with it with large frequent water changes. But to keep maintenance reasonable and the fish happy, I would go for a bigger tnak or smaller fish. Plenty of CA/SA cichlids which would love the 110 for a pair. Any that get up to 12" max should be OK, compared to the oscars' possible 16" or so.