We saw that when two subjects are combined together by and, we get a compound subject that takes a plural verb.
What happens when two subjects are combined using conjunctions such as or, either…or, neither…. nor?.
In such cases, the subject closest to the verb determines whether the verb is going to be singular or plural.
Oliver or his brothers are coming today. (brothers is nearest to the verb in the sentence, so it is plural)
My brothers or my sister is coming today. (sister is nearest to the verb in the sentence, hence singular)
My sister or my brothers are coming today. (brother is nearest to the verb in the sentece, hence plural)
This is also known as the law of proximity.