This is my take. Stack and Whack and OBW both are kalidescope blocks, in that a single design element is cut and repeated to form a concentric pattern. Both S&W and OBW can be made from Hexagons, octagons and/or squares. The set up technique is the same. Find the pattern repeats in your fabric and cut the number of repeats needed to make your block. In a S&W, the resultant blocks are set with contrasting sashing or other methods/patterns where the individual blocks are highlighted, as mimom has shown (Pretty Quilt!!). In a OBW, the blocks are assembled without sashing etc. The assembled blocks are placed based on the dominant color of the block. The individual block isn't highlighted. OBW are often Hexagons. Do a search for OBW in the picture area of this board. Easier to see a pic than explain. Clear as mud?