Oasis Hiroba 21 is a small skyscraper containing the prefectural government's iichiko Cultural Centre — a 1,966-seat concert hall and theatre named after the Oita-distilled shochu brand that funded it. The building also houses a public library and a free observation lounge on the top floor with a view across the city and out to Beppu Bay.
For travellers it is mainly worth a visit for the view (admission free, lift to the top), and for whatever is on at the theatre if classical music or Noh interests you. The plaza in front is one of the few sizeable open spaces in central Oita.