55 is possible. However the oscar will most likely touch both ends width wise. 75 is much more comfortable. After getting my 180 i cant imagine going back to 12 inch widths
ive seen an oscar grow to 12" in a 29 so i think the tank stunting thing is crap (with oscars anyway). reason i know it was 12" because its mouth was touching the front and its tail was touching the back and a 29 is 30x18x12... needless to say it was rescued.