This is the 515th most frequent Croatian word.
osijeku
"Osijeku" is the locative case of "Osijek," meaning "in Osijek," referring to the city of Osijek in Croatia.
In this sentence, 'Osijeku' is the locative case of the proper noun 'Osijek', denoting a location where the action 'studirao' (studied) took place.
Studirao sam na sveučilištu u Osijeku.
I studied at the university in Osijek.
Here, 'Osijeku' again uses the locative case of 'Osijek', showing the location related to the action of visiting.
Pozivam te da posjetiš izložbu u Osijeku.
I invite you to visit the exhibition in Osijek.
In this instance, 'Osijeku' describes the place associated with the action of leaving the documents, with 'u Osijeku' (in Osijek) being locative.