If the clause is essential (i.e., if the clause is essential to the meaning of the sentence) and the relative pronoun is the object of the clause, the relative pronoun can be omitted. The relative ...

Que comes before a noun or pronoun and relates to the object of the verb. When used as a relative pronoun, où can mean ‘where’. Où as a relative pronoun can also mean ‘when’.

The WH-phrase is called a relative pronoun in tranditional grammar ... but it may be a PRO-NP referring to inanimate objects when it is a relative operator: Which book did John read? *What did John ...