you don't get oil out of them, you infuse oil with them, getting any remaining thc to stick to the fats in the oil. same thing for cannabutter, only with butter instead of oil.
If you want to get oil out of stems you can extract them with butane. Collect a large amount of stems, like 5gs minimum. Look for a Butane Honey Oil extraction technique. It really is pretty easy and it will yield almost pure THC (and other cannabanoid) oil. Then you can use the oil like hash.