I'm working on a shed at home and, as I was ordering materials, found that OSB / chip board is much cheaper than plywood.

Why would an apartment building use plywood, when I imagine it'll be covered by insulation/vapor barrier anyway?

The main advantage that plywood has is that it generally tolerates getting wet and drying out better, especially at edges and seams and around fasteners. So while it still will have a vapor barrier and some kind of external cladding in front of it, it's used in my soggy neck of the woods instead of OSB just because it better tolerates moisture and humidity. Or it might just be a local code thing -- maybe Big Wood is lining the pockets of your local city hall officials :)

Oh! I know this! Yes its cheap. But a few years ago, Industry pushed really hard to get OSB fire code compliant for tall builds. That's why.

Rather than banning raw OSB, the modern building codes including updates under the International Building Code (IBC) and the National Building Code of Canada (NBC) implemented strict "Fire-Resistance Assembly" and adopted new rules for use of fire resistant treatments and build parameters for it.

Big Wood?