Oscars + Fire eel + tank barely large enough to hold eel = bad juju ... fire eels are scale-less fish, passive unless put w/ con-specifics, notoriously finicky eaters. And all around a joy to have in any tank and deffinately most peoples favorite fish or close to it of their personal stock... Fire eels do well with larger, non-aggressive fish. They are to big to eat, and won't compete for food generally. They are also prone to infections as much as clown loaches are. require either target feeding or live foods when first introduced ect.
aggressive biting fish are a big big mistake to mix w/ any of the eel fish. Oscars are big, aggresive, and nippy. him or the JD may not be able to eat a larger fire eel but they can and will beat it to a pulp and kill it from a secondary infection in all liklyhood unless you are running extensive filtration on a very stable system. If either of your Cichlids are showing aggression I wouldn't try it. If everyone was a 2" baby yet.. it might work. BUt mixing larger cichlids w/ more docile fish almost always ends in disaster.