This is the 559th most frequent Finnish word.
oulussa
"In Oulu" (a city in Finland).
'Oulussa' is used here as an inessive case to indicate the vacation location.
Tänä kesänä vietän lomani Oulussa.
This summer I am spending my vacation in Oulu.
'Oulussa' serves as an inessive to specify the place of the event.
Parhaat kalamarkkinat järjestetään Oulussa.
The best fish markets are organized in Oulu.
'Oulussa' is used in inessive to denote a destination in the context of completion of the action.