Can't compare OSBs to Berkshires, haven't had Berkshires, but I'd suspect they're pretty similar.
They'll need more food over winter, to keep warm, and depending on the conditions they may take less exercise - they don't like it icy and they don't like it too muddy - but so long as you watch their condition and don't overfeed, I don't see why they should be too bad, fat-wise.
I reared a late September-born litter of Saddleback x OSB over winter; they had the bull pen to themselves with a large round bale of hay for nesting, eating and playing. We sent the first off late Feb and he was beautiful, not fat at all. In fact my butcher said if I could produce them like that, but a little bigger (he was 55kgs deadweight), he'd buy them off me.