OpenXcom requires a vanilla version of the original UFO: Enemy Unknown / X-COM: UFO Defense (any edition) with the latest patch. It is not compatible with other games in the X-COM series. If you don’t own a copy, you can buy it from one of these retailers.
Do not use modded versions of X-COM, as they may be incompatible with OpenXcom.
Check the Installation documentation for more details.
This section is under construction. Use the nightlies in the meantime.