Verbal is from the Latin, where it means "word." Mainly it means "spoken" and is often contrasting with "written" as in, "The examination will have a verbal component as well as a written one. You can make a "verbal" agreement, but it is always better to get in writing.
Oral, on the other hand, is from the Latin word for "mouth." But to confuse things further, we can often use "oral" interchangeably with "verbal" as in, "The teacher asked for an oral presentation; the teacher asked for a "spoken" presentation! Oral is also used to mean "taken by mouth" as in, "The baby had to take the medicine orally. Be careful here as one cannot substitute "verbal" in this usage!
(August 28, 2010)