I never kept Oscars so someone else may have a better time answering. I am just going by what i know... which is Oscars grow fast and eat EVERYTHING.
I am not sure if oscars growth rate slows down if/when they get to a certain size, but when your 2.5" Oscar gets to 10", your 5" Dat still wont be. Also if you get a 8" Dat, then it will prob eat the small Oscar.
Someone with better knowledge of Oscar growth rates will prob be able to give you a better size combination to start with if there is one.
What I would do in your situation if i REALLY want the 2 is get the largest size Dat possible. Then grow out the small oscar in a different tank or using a divider, as it wouldnt take to long to get it to a size that the Dat wont try eating it. Then again, everyone else above says Oscar/Dat is not a good combo.