Oral: uttered by the mouth, as in oral testimony; using or transmitted by speech, as in oral methods of teaching languages; involving the mouth, as in the oral cavity; taken, done, or administered through the mouth, as in an oral dose of medicine.
Verbal: pertaining to words, as in verbal ability; in the form of words, as in verbal images; expressed in spoken words, as in verbal agreement.
Note: Many insist that oral relates to spoken words and that verbal does not mean oral. But verbal has meant oral since the sixteenth century.
Verbal has had the meaning “spoken” since the late 16th century and is thus synonymous with oral: He wrote a memorandum to confirm the verbal agreement. Slightly earlier, verbal had developed the meaning “expressed in words, whether spoken or written (as opposed to actions)”: Verbal support is no help without money and supplies. Although some say that the use of verbal to mean “spoken” produces ambiguity, it rarely does so. Verbal is used in this sense in all varieties of speech and writing and is fully standard. The context usually makes the meaning clear: No documents are necessary; a verbal agreement (or contract or order) will suffice. Oral can be used instead of verbal if the context demands: My lawyer insists on a written contract because oral agreements are too difficult to enforce.