It was almost certainly planned. The fans yelled out "OOC" and they start playing it very quickly after the yelled request. Most of us have seen U2 a number of times live and all of us have heard boots and they tend to take a short while in between planned songs (edge switches guitars, keys change, tuning,etc). To start playing a requested song seconds after someone yelled it out would be nearly impossible, it could be done but U2 don't really work that way. I could be wrong though.