Those are both very aggressive fish, so base on that alone not much if anything. Now base on size/ space requirements nothing. I have seen it posted here they need rather large tanks to be purely happy.
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The jack dempsey is practically done growing. They usually max out at 10inches. The oscar will only get bigger and are said to get to about 15inches but the biggest ive heard of was 13inches. Both fish are already really big and i personally wouldnt add anything unless you could get a larger tank.
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