The picturesque coastal town of Strömstad is only a two-hour drive up the lovely coast from Gothenburg. Or a quick hour and a half drive from central Oslo....
If one does not use or possess ones own boat, you have to get to Strömstad first and then take a ferryboat "Kosterbåten" to reach Koster. Departure from the north harbor in Strömstad, a pleasant crossing of around 40 minutes.