Reaction fire isn't changed in OXC or OXCE, however mods are able to make it work differently. For example, the original aliens had very high reaction scores, and so would usually get a huge reactions advantage over your soldiers. In mods the aliens may have similar reactions to your soldiers, or lower. Also, while only snap shots may be used for reaction fire, not all snap shots are alike. In my upcoming mod there is a defense turret weapon with a snap shot which is called "Auto Shot" and which fires three shots. Since it is internally a snap shot, it is used for reaction fire. OXCE allows weapon shots to be altered in many ways.
Melee reactions can trigger from moving in front of the unit, but not from attacking them from behind. I don't think attacking them from the front will trigger melee reactions, either. And performing melee attacks during your turn cannot trigger reaction fire at all. The aliens also have this advantage.