Jabes.plays.RPG says:
Speaking of Baric, he gets an opportunity attack against the female wight that passed him, yes?
By my understanding, they changed the OA rules from 3.5/4e to 5e. Instead of saying that you get an Opportunity Attack when you leave a space that a creature threatens, it says you take an opportunity when you leave a creature's Range. As I understand it, the Wight entered Baric's range (5 ft), but then never left it, and instead "walked" to the position she's in while remaining inside it that range.
This is how I've always done it while running 5e, ever since noticing that change. Though, admittedly, I've never looked up any official ruling or suggestions from other GMs, because I was pretty confident in it. I can do that later, when I have a chance.
Even if the rules hadn't changed though, the only reason the wight moved to that location was because of what IndustrialQueue wrote in his post, and the way in which he attacked. I originally had her on the map at I16, one space below Baric, and moved her after Baric's turn. I wouldn't have given an OA for it anyway, because I only did it for the cool writing :P