The word "οι" in the phrase "oι οποίες" is a definite article (feminine, plural, nominative case). We cannot use the word "οποίες" without the definite article.
The whole phrase "οι οποίες" means "which". Grammatically it is a pronoun which refers to the word "ιστορίες".
Here is the pronoun "o οποίος" in all genders and cases:
ο οποίος = which or who (masculine, nominative, singular)
τον οποίο = which or whom (masculine, accusative, singular)
η οποία = which or who (feminine, nominative, singular)
την οποία = which or whom (feminine, accusative, singular)
το οποίο which (neuter, nominative or accusative singular)
And in plural respectively:
οι οποίοι =which or who
τους οποίους = which or whom
οι οποίες = which or who
τις οποίες = which or whom
τα οποία = which