The spoken word conveyed with a rhythm and rhyme that helps an audience to remember the stories told by the speaker. The phrase "oral poetry" implies a passage of time and the lack of an authoritative work from which the poetry is based. Great works of Western literature like Beowulf were passed from generation to generation of an oral culture before being written down by observers.
Beowulf's Anglo-Saxon meter is actually a great example of a formal oral poetry that survived the test of time; lines like
ofer hronrade hyran scolde,
gomban gyldan. þæt wæs god cyning!
--lines 10 and 11 from http://www.georgetown.edu/labyrinth/library/oe/texts/a4.1.html
show the formalism. Notice the 'h' sounds repeated in the first line and the 'g' sounds repeated in the second. Beowulf's tricks like each line having four strong beats to a line, dividing each line in half with a caesura, and incorporating strong alliteration helped the audience remember the story for retelling.