You've got to bust through the plaster seal before you can drink. It's a pain in the ass. It was the second time I tried the stuff, so I knew what to do...bring it down to a local eatery and let management deal with it.
Rice husks. I'll have to do some more research on the topic of "oo", as the stuff is still a mystery to me. Alcohol is a liquid, but it seems like the contents are not the least bit moist. You add water, wait 5 minutes, punch the wooden straws through the mass (difficult!), and suck.
Tasty stuff. Sweet and sour. You might compare it to sweet sake or, if you're already familiar with Thai alcohols, "sato". I'd say that it's more complex than either, however. I detected hints of cinammon and/or coconut, though I doubt any was added. Apparently, you can get "oo" in pineapple and watermelon flavors too.
It's not easy to find oo in Bangkok. This pot was acquired in roundabout fashion: a couple weeks ago a taxi driver and I found ourselves chatting on the subject of oo. He actually went out, bought a pot, and stowed it in his trunk, waiting for our next encounter. I threw in a 40 baht tip for the effort. He told me he found the stuff at an "OTOP" (one tambon/village, one product) shop in Bangkok.
Befriended taxi drivers, by the way, are awesome resources. I can't count how many times I've hopped in a taxi, told the driver to "take me to a good Thai restaurant", and had a great meal.