coolant flush and is all I can think of, and warm water.

Fill it with a 50 percent purple power water mixture in it and let it sit for an hour. Than drain it out and flush with water.

If you have time, get a large gallon of purple power degreaser or equivalent, and add 25% water to it, then put the it in the radiator shake it well empty it repeat the process until there is no crud coming out.

this will help get rid of the oil, but It may leave behind baked on deposits, so use a radiator flush and distilled water on your first start up of your car, drive it around for a few miles return home dump it out, repeat if more crud comes out.

*Edited, didn't realize radiator was off the car.** So just take a garden hose outside, and stick it in where the top hose would go on the radiator, and let the garden hose blast pretty good on the inside of the radiator and let it drain until it has run clear for a long time and you're satisfied it's clean. You could do the solvents / degreasers others have said, but I'm not confident it would do much for you.

Run a good degreaser through the radiator a few times and rinse well.

Garden hose blow that shit out then pour in some gasoline drain that out then back with the hose one more time and your good to go.

Mineral spirits it's cheap and safe for aluminum. Poor it in shake it up and watch it all pour out. This stuff is perfect for oil. Some of it is flammable so be cautious.

Lots of responses here, lots of common sense as well. Thanks for all of the input!

If you think theres oil in it. Hillbilly solution: Put some gas in it, shake it around, let it set then run tons of water through it. It'll clean up