OSB can safely be used on layouts, but one needs to consider its nature and to use it, and to support it, sensibly.
I would be hesitant about using it for 'cookie-cutter' lengths of sub-roadbed, for example, but it should do well as risers to support the sub-roadbed. Tunnels portals and liners, as mentioned by Bwells, gussets, blocking, braces. If you would really like to economize and to use it for level surfaces that will have to support items later, make sure that no two supports for the OSB are further apart from each other than about 12", especially if it's only 1/2". And try to limit high humidity in that space that exceeds 65%.