DO NOT SOAK IN OLIVE OIL.
DO NOT USE A TOOTHBRUSH OR ANY BRUSH ON THAT COIN.
Unfortuately there are many that pop up with silly, non working methods of fixing a coin. Those are not of much use. Many Olive Oils are different since they are a manufactured item and some contain acids. Those will for sure not help your coin. And to suggest any type of brush on a coin is also not to smart. Ever look at the package a toothbrush comes in? Some are hard, some medium, some soft. Some have swirly things and some actually have small colored brisles. Way to many different ones so how could someone suggest something they know nothing about. Be real carefull listening to people that have nothing to loose but you do.
For one thing if your determined to attempt to clean that coin, start with the warm distilled water first. Soak only, no brushing. If nothing, you may want to attempt the Acetone soaking but if you do, do a search on this web site for Acetone. It is flamable and dangerous. If non of these work and you stil are determined to do something with that coin, you may want to try some of the Jewlery Cleaners from a
Walmart Store. The one for Silver only is the more powerful one and will remove lots of stuff from a Copper coin, BUT, remember it also will take off some of the original metal of the coin.
Someone gave me a large cent like that and I've been working on it for some time now. Not really a lot of improvement but nothing to loose.