There doesn’t yet seem to be a Category for the mobile app, but I found it by downloading from the Google Play site to my Android device (is it available on iOS?).
It comes with warnings that it is an experimental version, but I found it impressive in several ways and wanted to post something on OGS to point to it. My understanding is that someone outside of OGS is developing it with the permission of OGS. When you run it, you provide your OGS username and password just as with the desktop edition.
As a smartphone app, it is beautifully designed. It is simple, yet provides what I was expecting to support multiple concurrent games (I’m in the 9x9 correspondence game ladder), board customization, an easy Analysis mode, and most of the remaining necessary features of online-go. It is remarkable how well it fits into a small screen (I only tried 9x9 games). It even includes viewing your playing history as a graph over various time intervals.
I haven’t explored all of its features, and perhaps it has not even been announced yet, but it is a great addition to my mobile device.