Good question. No simple answer.
Since there is no quantified movment happening (or at least spelled out) - and the only word used to indicate movement is "switch places" I'd likely rule that there is no OA.
In the end, does this ruling break anything? No. Is it reasonable, yes. Is my interpretation unique in that sense that it doesn't follow forced movement or shifting? Yes. I can't think of too many other powers that cause simultaneous movement. The few that do, seem to indicate the movement of swtiching places just happens ignoring how it happens and what effects that movement has. Looking at the Compendium, the Power, Leaders Intercession, is an Immediate Interrupt that lets you switch places with an ally and take the damage of the hit. It simply ignores how it happens and the consequences of it. I'd say that is simply the best approach here.
Like I mentioned before, there will be considerable table variation with this one.
Edit - There is even one instance with an item where you and an enemy switch places. Trying to quantify the whole shift/slide, OA issue gets even messier. The more I think of swtiching places the more it seems that you exist at your starting point, and then exist at the ending point - almost like a teleport (not that I am claming that - just using it as an example of how this movement is similar to).