like UB says, get a length of something like 3/8 rod from the HW store, grind some flats on it so it fits the oil pump drive like the distributor does, chuck it up to your drill and spin it till it flows out the head.
It can take a while to get oil flowing to the rocker assembly. When it does it ain't gonna be a lot.
Anyways, do the drill trick.. no reason not to. I have always hated cranking motors with no oil pressure after rebuilds, these motors are easy to prime.
did you pack the oil pump with grease or vasoline? I did that one time because somebody reccomended it and it only clogged everything and made it super hard to prime. Never again. people might read that and think it was stupid.
edit: the distributor prolly fits tight because of the new o-ring.