That's a fairly low standard. Construction grade is frequently aspen flakes bound by polyvinyl adhesive. Aspen is by itself somewhat moisture resistant, to a greater degree than fir, spruce, white pine, and hemlock veneers. Higher performance OSB is usually differentiated by a much greater span and load rating, and the controls for chip size as well as heat/pressure treatment are more stringent. The guys and girls in Madison at U.S. Forest Products laboratories can be a great resource. Forest Products Laboratory | US Forest Service Research and Development
At a sufficient thickness, and with the appropriate bracing, it should be reasonably rigid and resist flexing, although not as well as furniture grade ply. There again, angle-spliced baffles and guides instead of flat sections could minimize the flex to negligible levels, and probably surpass plywood in flat sections, although that could become much more complicated to engineer for waveguide passage size and volume.
Consider glue blocks internally mounted in corners and tee-nuts for anything requiring secure fastening.
And you're gonna need a really good set of clamps, and some serious contact cement made for the purpose.
