I'd keep the oak dry, if you put any moisture on it you will lift the grain and possibly further mark the timber. Plus dried plaster is largely unaffected by moisture.

It is best to scrape it off with a proper wood scraper but you coul use a paint scraper (the sort used to clean paint off windows). Keep the blade almost vertical and scrape rather than cut the wood. You should get a very fine shaving coming off. If you were to just try to sand it off, it tends to push the plaster dust further into the grain.

Hope this helps

If it's water stain then you'll have a black mark on the oak. If you're lucky then it won't be too deep and you should be able to sand it out with very fine grade paper. If it has gone deep then you can, with care, remove it with oxalic acid although I have heard reports that, because it acts as a bleach, you can end up with a halo effect.

There's more info here http://www.ukworkshop.co.uk/forums/viewtopic.php?p=280501&highlight=oak+water#28...