I use a pump type oiler with a flexible spout,
I take a short piece of clear semi-soft plastic tubing and push it over the end of the oilers tip. As mentioned fuel line or other rubber hose will work, but then you can not see what is happening, like you can with clear tubing. Must be soft enough that it will squeeze against the surface to seal!
Clean really well around the ball oiler as this method will force crap into the oiler if it is there.
Place the end of the plastic tubing over the ball oiler, so that it surrounds the ball and seats on the outer ball housing forming a seal.
Pump the oiler, the clear tubing allows you to see the oil as the hydraulic pressure created forces the ball open and the oil is forced past the ball. After a few pumps you should notice that the oil is seeping out from under the ways, down the shaft or where ever else you are pumping the oil into.
Walter