14" fish, 12" tank = no room to turn around.
I used to raise oscars in 55's for profit, the idea was to get them from tiny to a larger size that commanded decent money. At about 6" (6 months) the fish were too big for the tank. Not to mention the need for extreme maintenance due to the fact that two big messy fish in low volumes of water just wasn't a good idea.
Many people will say that a 55 is adequate for an oscar, but IMO and IME 55 isn't even close. My suggestion would be a pair of Firemouths or angels really.
Dave