Roy here insists that the only way is to let each layer dry, if your using the paints thick this will take an age, although you can buy some quick dry thickeners to mix your paints with and also a lot will come down to what you mix your paints with full stop, he has his own secret reciepe mix of poppyseed oil, distilled turps and something else he wont say

and he reckons linseed is the worst for taking ages.
I did find today that you can get your base agent stuff as a quick dry option, but I only saw it for the artisan water mixable range of oils, so dont know if you can use it with normal oils.