Wood for boarding a loft?
For occassional storage chipboard or any plywood 1/2" or greater will be fine.  Go to 5/8 minimum if your joists are on 2' centers.

I personally don't agree with the advice to use plywood. Chipboard is fine: it's strong, and it's made using waste materials so it's more ecologically sound to use it where it's appropriate.  It's not appropriate for applications that are exposed to the elements in any way: in those cases plywood is more durable so justifies the 2x cost and the extra environmental impact.

Chipboard does have a strength axis, which you might as well take advantage of it: you want the joists running across the short side, i.e. 5 joists on every 4x8 sheet if the joists are on 2' centers.

Chipboard made for flooring has a tongue-and-groove along the long side.  That's because while the short sides are supported by the joists, the long sides bridge the joists.  Locking a tongue into a groove prevents sagging between the floor joists.  Obviously not a big deal for you storage loft, but if it only costs a few more bucks for the tongue-and-groove I'd get that.  I've only seen that in 3/4" and 1", BTW, so if you get that you'll have a proper floor up there that you could hold a neighborhood dance on.