Using or, what should be used next?
eg1. Mary or john is here.
eg2. Mary or john are here.
Or any other eg. you can say...
Only one of them is here, so we should use 'is'.
When we want to make it clear that we are speaking of only one, and definitely not the other, the we usually use 'either':
Either Mary or John is here.
If we are not so concerned with excluding one, 'either' is not so important:
(Either) Mary or John will help you.