This is the 732nd most frequent Greek word.
οποίων
"Of whom" or "of which" (possessive relative pronoun).
The word "οποίων" refers to the noun "φίλοι" (friends) and is used to indicate possession (their help). It is the genitive plural form of the relative pronoun.
Οι φίλοι μου, η βοήθεια των οποίων είναι ανεκτίμητη, είναι πάντα πρόθυμοι να με στηρίξουν.
My friends, whose help is invaluable, are always ready to support me.
The word "οποίων" is used here to relate "μαθήματα" (subjects) to "πλαίσια" (scope), implying that the scope pertains to the subjects in the genitive plural.
Το πρόγραμμα των μαθημάτων, στα πλαίσια των οποίων δουλεύουμε, είναι πολύ απαιτητικό.
The curriculum, within the scope of which we work, is very demanding.
Here, "οποίων" connects "κανόνες" (rules) with "κριτήρια" (criteria), illustrating a relationship of possession in the genitive plural case.