or

   has the grammatical effect of emphasizing the separateness of items rather than adding them together. If a grammarian offers you an apple, a pear, or a banana, he means that you may have one of them, not all three. Thus when or links two or more singular items in a sentence, the verb must always be singular. "It was not clear whether the President or Vice President were within hearing range at the time" (Chicago Tribune) should be "was within hearing range." If that sounds stilted, you can flag the singularity by inserting either ahead of the phrase ("It was not clear whether either the President or the Vice President was within hearing range"), or, more simply still, you can change or to and, thus justifying the plural verb. For a full discussion, see number².