I didn't mean pH, either. The oxalic acid in spinach is bound to iron as well as calcium. I have no idea how much unbound oxalic acid is in spinach, but it doesn't sound like adding calcium in the form of dairy releases the already bound elements. Some of the calcium in the dairy ends up bound by any free oxalic acid, so you lose the bioavailability of that without releasing what was already bound.
Does that make sense? Do we have a chemist here who could help us out? Skilletlicker, what was your investigation into basil about?