This is the 56th most frequent Greek word.
οποίος
The Greek word "οποίος" means "which" or "who" in English, referring to choice or identification.
The word 'ο οποίος' is used as a relative pronoun introducing a subordinate clause that provides additional information about the antecedent 'ο άνθρωπος' (the man).
Αυτός είναι ο άνθρωπος ο οποίος με βοήθησε.
This is the man who helped me.
The word 'ο οποίο' here is used as a relative pronoun referring to the antecedent 'ένα θέμα' (a topic), and it is also the object of the preposition 'για' (about).
Είναι ένα θέμα για το οποίο δεν θέλω να μιλήσω.
It is a topic about which I do not want to speak.
The word 'την οποία' functions as a relative pronoun connected to its antecedent 'Η απόφαση αυτή' (This decision) in the accusative case, giving more information about the decision.