“Που” or “ο οποίος”? Choosing the relative
The invariable relative covers almost everything — until the sentence turns ambiguous or a preposition is involved. That is where ο οποίος earns its place.
What “που” cannot do
“Που” is invariable: it shows no gender, number or case. As soon as a preposition must attach to the relative, ο οποίος is required: «το θέμα για το οποίο συζητήσαμε». You can keep που with a resumptive pronoun — «το θέμα που συζητήσαμε γι’ αυτό» — but that is markedly more colloquial.
- Η επιτροπή στην οποία ανήκει συνεδριάζει αύριο.The committee to which he belongs meets tomorrow.
Removing ambiguity
«Η αδελφή του φίλου μου που ήρθε»: who arrived, the sister or the friend? Ο οποίος settles it, because it agrees: «η οποία ήρθε» (the sister), «ο οποίος ήρθε» (the friend). That is the real argument for the variable relative in careful writing.