Prefectural BOE JETs have a lot of "supervisors." This is because although the Prefectural Office itself is located in Oita City, Prefectural BOE JETs can be placed all throughout the prefecture.
First is the English Teacher's Consultant (ETC) who works at the Prefectural Office, who acts as a sort of overall "supervisor" of the JET Programme for high schools in Oita Prefecture. The ETC handles most of the paperwork and bureaucracy involved with having ALTs in the high schools.
Then there is your on-site supervisor (or counselor), who is usually a Japanese teacher of English (JTE) at your base school. This person should be the supervisor who you can contact with questions, concerns, etc. on a daily or school life level. Their role is a little bit like a liaison for the ETC.
Most Prefectural ALTs also have secondary or tertiary schools that they also go to. Depending on your base school, the procedures for communicating with these extra schools are different. Often you may find that most of the communication is handled between Vice-Principals rather than your supervisor directly.