Like most pronouns, the relative pronouns who, which, and that have antecedents, nouns or pronouns to which they refer. Relative pronouns used as subjects of subordinate clauses take verbs that agree with their antecedents.

One of the

Constructions such as one of the students who [or one of the things that] cause problems for writers. Do not assume that the antecedent must be one. Instead, consider the logic of the sentence.

The antecedent of that is things, not one. Several things set us apart from animals.

Only one of the

When the word only comes before one, you are safe in assuming that one is the antecedent of the relative pronoun.

The antecedent of who is one, not students. Only one student was fluent enough.