it was kind of interesting actually, the oscar and my 12 inch sen never left each other's sides but the oscar would attack any other bichirs within eyesight.
http://www.monsterfishkeepers.com/forums/showthread.php?t=114484
This is my thread from Jan. I keep a 7 inch senegal with a 5 inch Oscar and they don't even bother each other. The Oscar gets greedy at feeding time and you have to make sure that the polly is getting food. I have the tank in my room for the relaxing effect and have not noticed a single ocasion of aggression. The tank only has a small cave for the polly, but it never uses it. Just make sure the Oscars mouth isn't alot bigger than the pollys head.
I have a tank with oscars that I would never even consider adding a bicher to, they would think it's the tastiest thing ever.....
...but I also have another tank with a ocuple of oscars and jd's that have no issues with the bicher I have in there....I'd say it's a personality thing...know your fish.