OBW can be either hexagon or octagon. The quilt's center usually has only one fabric, unless hollow cubes are incorporated into the design. Stack and whacks usually have sashing and use two or more different fabrics. Both are cut by carefully stacking fabric so that the design elements are lined up through the various layers.
OBW - books by Maxine Rosenthal
Stack and Whack - books by Bethany Reynolds
Stack and Whack has been around longer.