We have three main communication lists. Proceeding from most-to-least trafficked, geeky, and concerned with the code itself:
GitHub pull requests/issues: if you visit the OpenImageIO GitHub page, and click the "watch" button near the upper right, you will receive emails whenever a new issue or pull request is added. This is intended for people who actually care about the daily changes to the code itself.
oiio-dev is a medium-traffic list intended for general discussion about OIIO and its code, but not in as much detail as the GH pull requests (except in cases when a particular code change needs extended design discussion with the whole community).
oiio-announce is a low-traffic list just for new release announcements or other very important items of interest to anyone using OIIO.