What is the meaning of "O Laso"? Alan Wallace says this regularly, for example, at the beginning of this lecture: http://media.sbinstitute.com/courses/fall2012/13-loving-kindness-1/
Is it Tibetan? What is its direct translation?
From this transcript, it doesn't really have a meaning. Full quote:

Q: What does o laso mean?


That’s one of the great secret teachings I give out only to people who’ve achieved 8 weeks of retreat first. It’s very secret, have you received Tantric empowerment? O laso, you ready? Because once I have said it you won’t forget it, it’s one of those really, pointing out instructions - O Laso doesn’t mean anything at all. But it is Tibetan, so it is hard to find a phrase in a language that is definitely part of that language, because O Laso is definitely part of Tibetan, it’s not Hindi, or any other language that I know of, it’s definitely Tibetan, and it doesn’t mean anything at all. But it’s Tibetan and I don’t know quite how you translate it into English without losing its meaning, but it’s something like UM or how about this – WELL THEN , what does that exactly mean, “well then”? Well then, Ladies and Gentlemen. What exactly have I just said that imparted some information to you? (45:42) ‘Well then’ as opposed to ‘ill now’? O laso is kind of like that. It comes up a lot and it’s just become part of my speech pattern, and I haven’t seen any reason to break it, and also it sounds nice. O laso, o laso, and very often, because I have received many teachings from lamas, at the beginning they often say o laso, and that lets everybody know, hey, we’re about to begin, folks. O laso, so it’s a nice way to begin, it’s so smooth, almost like a mantra, o laso. And then when you finish, when you’ve finished something – o laso. Like that, okay? It’s a way to begin and a way to end.