4x8 and 3/4'' plywood.

birch is too expensive. ACX l dont know how much that would be, thats what l found on the metolius website. OSB is dirt cheap like 15 bucks a panel, but l dont know if that would last long enough.

any advise? thanks!

OSB will do the job if u really want to do this on the cheap. The only downside is that it does not handle moisture as well as plywood, & it will tend to splinter out. If u plan on keeping it indoors, put on a heavy coat of something---there is all kinds of coatings u can use, but avoid the slick coatings. Just make sure that the OSB r not fastened at points too far apart, so that it wo not flex much when u climb on it. l would recommend that u go with 3/4'' OSB, the bolt holes for the climbing holds will be less likely to crumble over time.

In the building industry, OSB is used for sheathing everywhere, except the places where the structural engineer specifically calls for plywood SHEAR walls. What l am saying is that OSB is pretty tough, even though it is very cheap.

Scythian is right, but still l would only use / have only used 3/4'' plywood.

OSB will wok if in a dry location if not apply a good sealer