Many less effective writers consider the words oral and verbal synonymous. These two words in fact have related but different meanings. Oral refers just to spoken language. But verbal literally means in words, which can refer to either spoken or written language since both types of language are literally in words.
Because the definition of verbal presents opportunities for confusion, more effective technical writers will avoid using verbal. They will use oral or spoken to reference spoken language and written to reference written language.
Oral references never use verbal because oral is more precise language than verbal. More precision in language makes for better technical writing, and that makes for a better presentation of both you and your brands to your audience.