Last week in San Francisco, Google showed me an app called Oz. Oz is a kind of augmented reality picture book: it places animated characters from The Wizard of Oz into the physical world, as viewed through a smartphone camera. I’d tried it a few months earlier at Google I/O, running on the Tango AR platform, and the content hadn’t changed. But the experience was far more interesting — because for the first time, it could run on a phone that I use every day.