I don't have experience with either of these, but I do want to try and help until someone who does comes along
You want the oscar to be at least the same length as it, and wider than it's mouth.
I'm not sure how wide their mouths get so can't really say if it'll be alright or not.
From what I've read it looks like they can eat stuff much bigger than them. The profile I'm looking at is on another forum that I don't think we can link here, so I will just quote from it.
"Gulpers WILL eat similar sized tankmates. Heck, they'll eat tankmates over twice their size given the opportunity. Gulpers will gobble gouramis, devour datnoids, bite bichirs, engulf eels, aspirate arowanas, chow down on catfish, peck at peacock bass, swallow silver dollars, and, well, you get the point. Unless tankmates are CONSIDERABLY larger than the gulper, they will not be safe. When I say larger, I do not mean just lenghttwise. There have been documented cases of gulpers as small as 3 and 6 inches eating arowanas as large as 8 and 13 inches, respectively. For a tankmate to be safe with a gulper, it should be at least as long as the gulper, and wider than the gulper's mouth. Even still it is not necessarily safe."