The calculated distance (air line) between Haltern am See and Oer-Erkenschwick is approximately 8 miles respectively 13 kilometers.
By car or train, the actual journey to Oer-Erkenschwick is certainly longer, as only the direct route (as the crow flies) between Haltern am See and Oer-Erkenschwick has been calculated here.
Air line (approximately)
13 kilometers
7 nautical miles