it is best to remove the old floor and replace with the new. if you install over the existing flooring, the new will be higher and you will need transitions to the new. my house had the kitchen, dining and halls done over existing floor with 3/4 in prefinished. the baseboards wer left in place and 3/4 inch corner round used for shoe. it is easy to trip going from the carpet to the hardwood due to the offset with no transitions. and in my opinion looks very crappy.
if i could afford to replace 800 sq. ft. of oak, i would rip it out, the old floor also and install correctly.