This is the 1226th most frequent Polish word.
opolu
"Opolu" is the locative case of "Opole," which refers to a city in Poland.
In this sentence, 'Opolu' again uses the locative case to denote the place associated with the festival.
Festiwal w Opolu odbył się na placu przed ratuszem.
The festival in Opole took place on the square in front of the town hall.
The locative case of 'Opole' ('Opolu') here designates the location where the person lived in the past.
Kiedyś mieszkałem w Opolu przez kilka lat.
I used to live in Opole for a few years.
The word 'Opolu' is the locative case form of 'Opole', used here to indicate the location where an action took place.