The term "open grain" refers to the size of the pores in the wood.
If you look closely at the image of this board, you can see the individual pores in this piece of red oak.
Now, take a look at this board - especially pic three - you will not be able to see any pores - this is known as "closed" or "tight" grained wood.
Open, or loose grained woods are frowned upon for use in end-grain cutting boards because liquid can easily soak into the pores - this will not only rot the wood quickly but can trap bacteria inside.
Red oak has such open grains that if you were to puddle water onto a five inch thick piece of end grain red oak and wait around ten minutes you would see it drip from the bottom - give it a try!
Because of Red Oak being an open pore wood, this cutting board is in my kitchen primarily to sit on the counter top! But we do cut bread and dry vegetables (lettuce, beans, etc.) Absolutely no meats will be cut on this board.
I watched a video from woodwhisperer.com http://thewoodwhisperer.com/butcher-block-cutting-board/ I found it really interesting. Most cutting boards have a butcher block oil finish but Marc talks about using a very thinned varnish on his cutting boards. The goal is not to make the varnish build up. Only put it on so it soaks into the pores. I used this method on this red oak cutting board.
General question for an open-pored wood like oak-if finished with a penetrating/curing/sealing oil like BLO or tung oil, does that mitigate any of the porous "risk"?