Hi and welcome to the forum!
Oscars can be rather boistrous. Yes if grown on from a young age they can get along but IME with both these fish they havnt got along. My Oscar is getting big, and bigger each day and is boistrous and doesnt tolerate anything in his tank and subsequently has his own tank. My male EB Jack is rather timid and goes with any fish i've ever added to his tank.
In the size tank you are planning i would say it could be possible, but as with any larger cichlids you just never know untill you've got the fish and they've grown into their personalitys. Each fish is unique and can react completely different to another same size/species fish.
A good point raised by mark. Your Oscar would hit 6-7" after a year maybe even 8". Whereas a Jack will hit 6" after 2 years nearly. So getting the EB first and giving him a year or so to grow would be better, or buy a bigger one straight out. Although that can be expensive as my 1.5" juvi cost me £25 in 2008.
