How do you get the oil out? I assume some kind of pump?

With a plastic barrel your options are kind of limited. You could weld expanded metal to a metal band that will set just under the lid, put another metal band on the outside and screw them together through the barrel. This assumes you have access to the other half of the lid where you could cut a hole through the expanded metal under some sort of lockable lid.

A metal drum would have been easier. Do you ever take your used motor oil to the auto parts store to dump it in their container? Think along those lines. No need to reinvent the wheel.