Yes, but get your CK first once the CK hits 6-8 inches then get the Oscar but get the Oscar at 3-4 inches that is what I did and so far it has worked out well-reason to get CK first is that Oscars grow fast-I mean fast.
This is possible but the CK will get twice as big as the oscar. Not a big deal, because once the CK hits 12" it slows down the growth. I just wanted to clarify that the CK can get to 20+. And I have seen them that big in person so I know its true. The pic is of a CK in a 29 gal at my lfs. The CK is from SA not Asia and Africa.
Sould not be a problem. Knives are typically only aggressive toward their own kind and fish they think they can eat. Oscars should be OK. I raise my baby clowns with angels and discus up until they are about 7-9 inches and they do fine.