Plywood vs OSB for Climbing Walls
Oriented Strand Board (OSB) is made from rectangular wood chips glued together using a heat-cured adhesive. The rectangular wood chips form layers. Each layer is oriented at 90 degrees to the previous layer. This creates a similar effect to plywood's cross-lamination process. By comparison, plywood is made from continuous thin layers of wood that are cross-laminated and glued together. Each layer is laid at a right angle to the layer above or below it. There are always an odd number of layers to plywood, so each direction has an equal number of ply layers.
Comparison of Plywood and OSB
Plywood is about twice as expensive as OSB. For this reason many homebuilders use OSB for sheathing and decking. OSB has a higher shear strength than plywood, so when nailed or screwed to floor joists, or roofing studs OSB is a good choice. Also, there are no soft spots in OSB. A soft spot is a location in plywood where a know occurs in the ply. At that location there is one less ply, and sometimes by chance two or more knots may occur at the same place.
Unlike roofing or flooring, a panel used in a climbing wall also must hold a t-nut securely in place. If the t-nut loosens the hold will spin. Also, a climbing wall panel also has forces from the climber, not just the static weight of the structure itself. With these considerations, let's compare OSB to plywood.
Screw Pull-through Strength Comparison
Tests comparing pull-through strength between plywood and OSB indicate that 17 pounds of force are required to pull out a nail from plywood, but 15 pounds of force from OSB. This may be insignificant in itself, especially considering the number of screws holding a single panel to the studs. Nevertheless, in terms of nail pull-through strength, plywood is a little stronger than OSB.
For a climbing wall, it's not just the pull-through of the screws to think about. I have seen t-nuts pull through old plywood walls. Considering the pull-through strength of OSB is about 11% less than plywood, it is common sense that this would happen more frequently in OSB as the panel ages.
Effects of Moisture Comparison
OSB tends to hold more moisture, compared to plywood. This could lead to an increased potential for fungus or weakening.
When exposed to moisture, OSB swells more than plywood. Some sources claim OSB can expand up to 15% when exposed to moisture. The greatest swelling on an OSB panel is around the edges, where cuts expose the layers. This would include the holes for t nuts. Through the course of time, moisture/humidity will cause the edges around the t-nut openings of an OSB panel to swell, and the outer edges to lift slightly.
For a climbing wall, swelling around the t-nuts of an OSB panel could loosen the hold and cause a spinner. If texture is used, the moisture from the texture mix will take much longer to exit the OSB, compared to plywood. Swelling of the OSB panel may cause cracking of the texture, especially at the joints between panels.
Other factors
OSB weighs about 15% more than plywood. This comes to about a 10-pound difference per panel.
Plywood is more dimensionally stable and has greater stiffness/less flex than OSB. An OSB wall may have a tendency to squeak simply due to its slightly higher tendency for flex.
OSB has a smaller dimensional tolerance, meaning you need to leave a slightly greater gap between panels to allow for expansion/contraction.
Conclusion
Some Internet sources recommend OSB for home climbing walls due to the cost savings (half the cost of plywood). In my opinion, plywood is a better choice. By comparison, using plywood will give less spinners caused from lifting around the t-nut holes, and a little less squeaking as you dyno and move across the wall. If you texture, you will have less cracking due to the swelling.
Also see:
Rise-Run Online Calculator
How to Install T-Nuts
Climbing Wall Texturing
How to Make Climbing Holds