I moved it, sorry. D: I need to look up some names, since "that bit with the piano you hear early on" isn't necessarily very helpful, but that. It's come to symbolise D:OS for me more than the main theme did. And the bit with the Russian(?) woman singing when you're in the homestead, which has a sort of peaceful melancholy is a particularly atmospheric and poignant one as well.
Edit: oh, that second one will be the Mysterious Guest song you linked to. Awesome.
Edit II: and "the piano one" is actually next up at 14, "The Power of Innocence". linky