Make sure that you put some oil in the engine !
Don't forget that !
Now that you have put oil in the tank, ( oil being lighter than the coolant ) it will be locked at the top of the tank.
I would remove the rad with the tank, use a water hose, with some dish soap clean it up. reinstall, fill it up, and go.
If you have started the car already, well.... the coolant will actually mix with oil
and make nice brown looking slurry, it is still cleanable just need a lot and lot of flushing ( usually with special detergent )