I've seen people do two Oscars in 5 foot tanks, total volume varying; but if it was me, 5 foot length and 18-20inch back to front would be the minimum footprint.
10 yrs ago I had 2 oscars in a 5ft 125 tall. I ended up transferring them to a standard 6ft 125. They did fine in both at 12" sl with one 75% weekly waterchange. Now bigger is better I agree. However they both had plenty of room and water quality was very manageable. I dont overfeed though.