I don't know how much truth there is to this but a local Oscar breeder told me that it's best to keep them in odd numbers. it has something to do with forming pairs when they are young. We have two that have been inseperable since we braught them home.
Oops...I just realize how strange that picture must look to Oscar owners...thats the Angelfish tank and the Oscars were only in there for a short time...even so they managed to pull out a good half of the plants. Their new 100 gallon digs is all rock...lol