I think most loaches have a deffense to Oscars that make it nearly impossible to go down their throut if they are about the same size. There is a down fall.. Sometimes Oscars attack the Loaches untill they remove them from the face of their tank.
My Oscar seemed fine with a herd of 9 clown loaches kept with him in a 125 gallon tank. Some were definitely small enough for him to eat, but I never saw him try.
Maybe he had a go at one of them, and didn't care much for those nasty little eye barbs? :?
I think it really depends. I had an Oscar and a Clown Loach that got along very well (as good as could be between the two of them), and then one day I woke up and he was missing. I think it depends a lot on the personality of the fish. Sometimes it works, sometimes it doesn't.