Ooooh bad. Do not do this. First, you'd have the neck-freezing-and-possibly-exploding scenario mentioned in another answer. But as an added bonus, liquid oxygen causes rapid oxidation of organic material. This is a fancy way of saying it's going to start your mouth on fire and then blow it up. Literally your mouth will start on fire. On Fire. And then it will blow up. It's hard to believe I'm typing these words in order: your mouth will explode. But there it is.