Adding an oscar to a cichlid tank?

Depends what size tank and what stock of cichlids...guessing from the tanks in your signature, an Oscar would not fit well...with tankmates you'd want at least a 75g or larger...
 
There are an awful lot of cichlids; assorted cichlids like in your sig makes me think africans, and I would say no.
 
water requirements are different, food requirements are different (oscar - meat, africans - veggies), space requirements are different - oscars get aggressive when overcrowded, while africans become less aggressive when properly (heavily) stocked. the oscar will bite, chase, and beat on the africans, while the a's will probably nip the oscar to death if it is small.
 
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