Juan is one of the employees who work long hours.
[Several employees work long hours. Juan is one of them. The plural word employees is the antecedent of who, the subject of the plural work in the relative clause.]
Juan is the only one of the employees who works long hours.
[Only Juan works long hours.]
Agreement of Verb
Restrictive and Nonrestrictive Clauses
Clauses with Quantity Words
Clauses with Prepositions
That as Relative Pronoun
Position of Relative Clause
Where and When