Heading towards the sea from the north exit of Oita Station, you'll find a plaza called Shukusai no Hiroba (Festival Plaza). This event space was built on the former site of a commercial building and is equipped with a movable roof.
It seems the benches and other seating can be moved around, and on the day I visited, many seats were set up in preparation for an event.
You might find different seating arrangements depending on when you go. If there's no seating available on the day you visit, I apologize in advance...