Most people use phrases like “verbal agreement,” “oral exam,” and “verbal abuse” without really stopping to consider what they mean. As copy editors, we don't have that luxury.
Sometimes, as these examples show, oral and verbal are used as synonyms. But should they be?
Purists (aka Grammar Nazis, errorists, grammaticasters) will tell you that verbal refers only to being composed of words (or, for linguists, of verbs) and can never be used simply to mean “spoken.” If you mean “spoken,” they'll tell you, use oral.
But such a black-and-white pronouncement leads to problems in the vast gray world of interpersonal communication.
Some things that are oral have nothing to do with the spoken word
In the instances when one must choose between oral and verbal, the thing is likely both. For example, an oral exam is also a verbal exam because it deals with words — in this case, spoken words.
Unless, of course, you're at the orthodontist, where an oral exam involves an up-close-and-personal look at your pie hole, and oral history means something different from your family stories. Oral means more than just “spoken”; it means “of or pertaining to the mouth,” and because it can be applied to a much wider area, oral offers a greater opportunity for (sometimes embarrassing) misunderstandings than verbal does.
People have used verbal to mean “spoken” for centuries
As Gabe Doyle noted in a post on Motivated Grammar, “the first attestation of verbal meaning 'conveyed by speech' comes in 1617,” according to the Oxford English Dictionary, and it has been used as such ever since. In fact, based on Google Ngrams results, verbal agreements and contracts were more common than oral agreements and contracts until the first decade of the twentieth century. I can't account for the shift in the 1900s, but it's worth noting that all four forms are still used regularly and fairly interchangeably, and the likelihood that all — or even most — of those verbal agreements and contracts were written down seems pretty low.
What's an editor to do?
As I've written before, etymology tells us where a word came from, not what it means. Words are defined by how people use them.
In the case of oral and verbal, there's no simple rule. Editors must be aware of the sometimes subtle differences these words can present and the possible misreadings that may follow and then make the best choice.
For example, there is a clear difference between an assessment test that has a verbal section and one that has an oral section. Referring to someone's “oral and written skills” may be clearer and more exact than talking about their “verbal and written skills,” which some will label as redundant.
But on the other hand, verbal abuse becomes oral abuse only when someone either starts biting or takes a hit to the mouth.
Grammaticasters may try to restrict verbal to meaning “consisting of words,” but English usage has made it synonymous with, and sometimes a better choice than, oral.