OSB is Oriented Strand Board. It is building product similar to plywood, but made from wood chips(scraps) glued together into sheets. OSB is often used in place of plywood because it ismore economical.

"peacebypiece" is correct...you can buy it at home improvement stores. We secured the completed ironing top to the dresser with removable adhesive putty, or you can fashion an open frame on the bottom to keep it secure and/or to raise the height if necessary.

My dad made my mom one too, but instead of canvas, he used that silver ironing board stuff, which is now transfering to her fabric when she starches.