I'd scrape with a one-sided razor blade. Find blades at any hardware store.

Chemicals in the little plastic feet that keep it soft and flexible can get into the finish doing the same thing. This is known as "plasticizer migration." Sometimes, it can add dark color, and sometimes, it just eats it away, down to bare wood. You can try some light abrasion like 0000 wool. You might also try "stain remover cloths" that you can find at paint stores or in furniture care departments. Guardsman and Jasco both make these. Sometimes this works, and sometimes it doesn't. Sometimes you can touch up the damage and sometimes, it needs to be stripped off. There is no pat answer, you just have to try and see what works or what doesn't.

Before you do that (they can take finishes off, but most often don't) try to soak it in vegitible oil (Just dab a bunch on the spot) and then gently rub off with a scruffy pad that won't damage the surface. It works amazingly well.

I don't really have a solution on hand for you. But there are several lessons here: 1. Oil has virtually no protection against moisture. In fact some oils are hygroscopic (meaning they have less than 0 water resistance, actually attracting moisture). 2. Wax has virtually no protection against moisture. As one finishing expert writes, "Wax is the closest thing to no finish at all." .3. Don't believe everything you read on the internet. OxiClean is sodium percarbonate, that when mixed with water, has a chemical reaction that creates hydrogen peroxide. That is a bleaching agent and why it works on things like clothes. When you are using a new product or process, ALWAYS test out on a piece of scrap of the same material or in an obscure spot. 5. Wood bleaches are not "spot cleaners." Most are designed to be used over a whole piece, and on raw wood. http://www.woodcentral.com/bparticles/woodbleach.shtml Like I said, I don't have a good answer for you. Oaks are tannin-rich wood. Chemical reactions between tannin and iron can create black spots like this. Might there be iron in your tap water? The article above talks about oxalic acid as a way to remove this, with all the above warnings on its use. I'd be tempted to just leave it and not make anything worse.