The calculated distance (air line) between Dortmund and Oer-Erkenschwick is approximately 12 miles respectively 20 kilometers.
By car or train, the actual journey to Oer-Erkenschwick is certainly longer, as only the direct route (as the crow flies) between Dortmund and Oer-Erkenschwick has been calculated here.
Air line (approximately)
20 kilometers
11 nautical miles