now what I am about to say is all dependent the fact that you want maximum growth and efficency, if you just want to get rid of oats then by all means soak away.             i'd wait till they are at least about a 100lbs, unless you were just going to give them no more than a pound a day, but you said you had tons, so I would assume that you were just going to pour  in a trough what they are going to eat in one day.  
 
    oats have a pretty low energy level compared to corn, so the smaller pigs can't eat enough to get the energy they need for optimum growth. over a 100-125 lbs they can eat a lot more, and make up any energy shortfall by consuming more feed.                   the only things soaking oats brings to the table is that pigs prefer their food moist and will increase consumption, and will help some what with digestibility.   and also beings that pigs only have a mono gastric digestive system, they have a tough time breaking down cellulosic feed stuffs of which oats have a greater amount than other grains.       oats use in pigs is pretty much limited to a filler and stool conditioner in breeding, gestation, and farrowing rations.   and to a lessor extant in weaner rations.