I had 3 Oscars (2 tiger and 1 red) that lived very peacefully with a 17" Clown Knife, Pacu, Red Devil and a Jack Dempsey. They all seemed to get along just fine. I think as long as there is plenty of food and room and homes they will all ignore eachother. They did tend to pick on the Pleco's though.
my concern would be the oscar rather than the clown knife as my experience with oscars is that they can attack tankmates for any reason at any time and I think they are quite unpredictable in that respect. I keep a clown knife and I don't think he is as aggressive as an oscar
It all depends on tank size, in an average sized tank it is likely there will be problems as both fish can be territorially aggressive but in tanks of 180 gallons or more the aggression is less of a problem.