You might mean the fact that OpenXcom is not a direct port of the original source code, as that was never released. Instead, it is completely rebuilt from scratch and partly reverse engineered. But, to my knowledge, still all of it functions and looks exactly as it did in the original, minus bugs, and with the only exception being bigger alterations to the AI as was mentioned. Although even that is kept as close as possible to the vanilla game.
OpenXcom has all but replaced the vanilla version of the game, as it stays true to it in every major way, yet greatly improves on the controls/handling and many more aspects. Also, it features a wealth of mods by now, as well as builds for different systems. There's even a terrific Android port!
If you've never played X-Com, I'd suggest you start with the GOG version to see what the game is like and get a feel for it, and then after a little while switch over to OpenXcom. You will probably want to start the game over a few times until you got the hang of it, anyway, so there's no harm in trying both versions and switching between them :)
Post edited February 01, 2018 by Syrion