Oriented strand board, or OSB, is an engineered wood product formed by layering strands (flakes) of wood in specific orientations. In appearance it is has a rough and variegated surface with the individual strips (around 2.5 by 15 cm each) lying unevenly across each other in the direction of their grain.

It is manufactured in wide mats from cross-orientated layers of thin, rectangular wooden strips compressed and bonded together with wax and resin adhesives (95% wood, 5% wax and resin). The strips are created by shredding the wood into the strips, these are sifted and then orientated on a belt. The mat is made in a forming bed, the layers are built up with the external layers aligned in the panel direction and internal layers randomly positioned. The number of layers placed is set by the required thickness of the finished panel, typically around a 15 cm layer will produce a 15 mm panel thickness. The mat is then placed in a thermal press. Individual panels are then cut from the mats in standard sizes.

Different qualities in terms of thickness, panel size, strength, and rigidity, can be given to the OSB by changes in the manufacturing process. OSB panels have no internal gaps or voids, and are water-resistant (though they do require additional membranes to achieve impermeability to water). The finished product has similar properties to plywood, but is uniform and cheaper. It has begun to replace natural plywood in many environments. The most common uses are as sheathing in walls, floors, and roofs.

In the United States the standard thickness of OSB is 7/16".

While OSB does not have grain like a natural wood, it does have a specific axis of strength. This can be seen by looking at the alignment of the surface wood chips. The most accurate method, though, for determining the axis of strength is to examine the ink stamps on the wood placed by the manufacturer.

There is some debate over the environmental impact of OSB. On the one hand, it allows producers to use tree species that are otherwise unfit for standard veneer plywood or lumber -- species like aspen or poplar. However, the manufacturing process requires the use of a variety of volatile compounds including formaldehyde. The production method uses almost all the wood of the harvested trees and both small, young trees and lower quality fast growing species can be used. In 2001 19.4 million m³ of OSB panel was produced in the USA.

Because natural plywood is made of layers with alternating grain directions, it can be difficult to cut. OSB, while lighter and easier to cut than the natural equivalent, leaves a gummy residue on saw blades from the waxes and adhesives used during manufacturing.

See also: Pressed wood.