Oral means "spoken rather than written" and "of or pertaining to the mouth." Verbal means "associated with words." Verbal can and does refer to what is written; oral does not. Be careful in using oral and verbal with words like agreement, promise, and understanding. If the agreement (promise, understanding) is not in writing, oral makes that sense clear. That is, verbal is less precise than oral in conveying the idea of "by mouth." When you can choose between a word that means two things (verbal) and one that can mean only one (oral), try to be precise. Why not use oral and written for clear contrast? Aural, a rarely used word, applies only to the ear and the sense of hearing: "His aural sense is defective because of a damaged eardrum."
Dictionary of problem words and expressions. Harry Shaw. 1975.