Google Docs does use OT. Or at least it did about six years ago, and I doubt that has changed. Source: I cowrote the original implementation (which was not OT, it was a three way merge of complete HTML files), and I was around when a new team rewrote the entire app using OT.
Historical note: live three way merging of HTML files, being produced by content-editable div implementations in different browsers, which like to rewrite each other's coding decisions in arbitrary ways (suddenly all your BRs become Ps, whee!) is "fun" in a way that I don't really ever need to revisit.
Historical note: live three way merging of HTML files, being produced by content-editable div implementations in different browsers, which like to rewrite each other's coding decisions in arbitrary ways (suddenly all your BRs become Ps, whee!) is "fun" in a way that I don't really ever need to revisit.