Hey,

Oscar  fish are very popular for home aquariums, but they’re not to be purchased without thought. They’re infamous for their aggressive and territorial behavior which can make them difficult to handle. ...

The Giant Gouramis are generally good community fish. Somewhat loners, they may fight fish of the same species when they are young. As they get older they mellow out but males tend to be aggressive to one another and may fight, rendering the well known gourami “kiss” in an aggressive ritualized manner.

The conclusion is oscar is more aggressive than giant gouramis .