Traditionally, oral means spoken (an oral examination) and verbal means related to words (verbal reasoning).
But 'verbal agreement' is the idiomatic term for one that is spoken, not written. There is no ambiguity here since an agreement necessarily involves words.
Another idiom is 'non-verbal communication', the use of gesture and body language instead of words; effectively this means unspoken since it is only used of people in face to face contact, who can observe one another's physical behavior.
Source: Your Dictionary