If OUAT is before the fireworks, then there will be a crowd already gathered. If OUAT is after HEA, then it does clear out considerably. You should be able to get an excellent spot right before showtime.
I do like OUAT, but it is nowhere near as good as HEA. It is almost entirely projections with a couple moments of pyro. Since I’ve seen it already, I don’t plan my schedule around it, but if it works out to be in front of the castle then, I’ll stop for it.