This is the 178th most frequent Greek word.
οποίοι
"Οποίοι" means "which" or "who" (plural, masculine or mixed gender).
Used as a relative pronoun to introduce additional information about 'the friends'.
Οι φίλοι, οι οποίοι επισκέφθηκαν το μουσείο, ήταν ενθουσιασμένοι.
The friends, who visited the museum, were excited.
Used to specify the exact group of 'students' referred to.
Αυτοί είναι οι μαθητές οι οποίοι κέρδισαν τον διαγωνισμό.
Those are the students who won the competition.
Used as a relative pronoun to connect 'the topics' with the actions described.