The most striking difference to OXC appears to be that the AI is much more passive (moves less and tends to set up ambushes more).
Especially in the early missions, the aliens were camping a lot more at certain spots in the escort USO. They didn't necessarily even move at all, making spotting them unreliable with particle disturbance sensor. You could also see this with the terror missions. Instead of moving a lot and going on rampage against civilians (or the X-COM), the aliens kept much more indoors or near to their spawn locations.
With the exception of ambushing, in general OXC behavior seems to result in a deadlier missions. As a consequence, the original AI behavior also likely results in very tedious bughunts (I can certainly remember some OG e.g. shipping line terror missions, alien colony 2nd phase). With OXC AI, the aliens almost never stay completely still (except when panicking) so you can almost always spot the aliens with meticulous usage of motion scanners.
