The calculated distance (air line) between Donostia / San Sebastián and Oñati is approximately 29 miles respectively 47 kilometers.
By car or train, the actual journey to Oñati is certainly longer, as only the direct route (as the crow flies) between Donostia / San Sebastián and Oñati has been calculated here.
Air line (approximately)
47 kilometers
25 nautical miles