It's not that I like to contradict authority figures...sometimes, I
love to. Oral literature is one thing, sure, but let's not forget that literature itself implies writing, as the word comes from the Latin
littera, or letter. To drive the point home, here's an example: if someone recites the greatest example of oratory ever known to man, is that person therefore qualified to receive the Nobel Prize in Literature? :) "Written literature" is like a musicologist saying "concordant harmony" --- it is redundant! It's somewhat sad that such a misnomer has burrowed its way into our language.
But, anyway, now I can see what you are asking. This is going to require some research on your part, of course, but here is what I know. Oral literature is anything memorized and recited: songs, epics, myths, legends, traditions, old wives' tales, bedtime stories, jokes, modern-day urban legends, etc., etc. Take any of these and compare them to
literature (I'm not going to say "written literature"!) and you will see an informality and ease about them that is often difficult to translate or distill.
Hope this helps!
Ned Plimpton