The figure shows a pyramid OABCD in a coordinate system with the starting point O. Determine the angle which is made out of one of the pyramids' bases and the base OABC.

I hope this is understandable, my english isn't really that good, due to teh fact that i'm from Denmark

The points are A(4,0,0), B(4,4,0), C(0,4,0), D(2,2,10) and O(0,0,0)

Can someone please help me?