I don't know if it's still possible on the newer ones but back in 2012 they figured out how to turn it off completely on paperwhites
http://www.mobileread.mobi/forums/showthread.php?t=200266
Here's a thread on how to.
The Oasis having a light that turns off makes me want to get one but they're £230 and that's a no joke price.
You could get a Kobo Aura H2O for 50 bux less or the Kobo Aura One for even less and not have Amazons nonsense and from what I've read the light does shut off completely at 0% brightness - at least on the One and I'm fairly sure the H20 does too.