I've used old flooring boards for things like making a table, and oil stains on timber can go pretty deep. Some boards I used were sourced from an old workshop and had various oil stains. Even taking off several mm with a thickness planer the oil stain is still apparent. Patina, they call it
Anyway you might find you can't remove it. I don't think this is necessarily a problem though. I have had success putting seal coats over clean albeit visible oil stains.
I would be less certain about using a dehumidifier unless you're sure the wood is unduly wet. Is that likely, given that long