The calculated distance (air line) between Ou Bei Choan and Siem Reap is approximately 81 miles respectively 130 kilometers.
By car or train, the actual journey to Siem Reap is certainly longer, as only the direct route (as the crow flies) between Ou Bei Choan and Siem Reap has been calculated here.
Air line (approximately)
130 kilometers
70 nautical miles