IMHO in a 75 I would not add another cichlid. JD get to be like 10 inches. Rule of thumb 1 Oscar needs at least 55 gallons of water alone. If you want a tankmate I would get a Cat...maybe a Raphael or a Featherfin. I have both in my 90 with my 6 to 6 1/2 Oscar. They're great cats and very cool looking and will help you clean the bottom of your tank late at night. Cool to watch.
I agree with funnyjew2. Not sure about the Raphael (Though they will certainly hide well during the day, and avoid the worst of your hungry Oscar's tricks). Definitely, a larger african Synodontis type catfish, will usually do the trick when stocking with grumpy 'tough guy' cichlids.
Or, if you just have to have another cichlid in there? You may want to consider a Severum. They are just a bit smaller growing than the other choices, and they tend to mix well with Oscars for some reason.