The calculated distance (air line) between Frenštát pod Radhoštěm and Opava is approximately 30 miles respectively 49 kilometers.
By car or train, the actual journey to Opava is certainly longer, as only the direct route (as the crow flies) between Frenštát pod Radhoštěm and Opava has been calculated here.
Air line (approximately)
49 kilometers
26 nautical miles