We will be holding an intensive, retreat-style workshop on OH as a probe of the dark interstellar medium. We hope to welcome you all to the Southern Highlands of New South Wales (Australia) during the week of April 16-20th, 2018.

It is an exciting time to be working in this field. As interest in CO-dark H2 grows, OH has emerged as an promising probe of the diffuse molecular (or "Dark") ISM. With a rich literature dating back to the earliest days of radio astronomy, a number of survey projects either completed, planned or underway, and a growing library of models and analysis techniques, now is the perfect time to take stock of where we are and where we wish to go. To this end, this workshop will bring together an international group of experts on the observation, physics and chemistry of interstellar OH, and on the Dark ISM in general, to see how we can jointly address the following questions:
 

• How effective is OH as a Dark Gas tracer on large scales? What are the observational and theoretical hurdles still to overcome? How does it compare to alternative spectral line probes such as CI and CII?
• How well do we understand the excitation physics of the 18-cm OH ground-state lines? How critically do our assumptions impact our ability to interpret observations?
• How well can non-LTE models constrain the physical conditions in OH-bright gas?
• How good is our understanding of the chemistry? How well are chemical models able to constrain OH abundances in different regimes?
• With the exception of strong masers, is there any hope of studying OH in external galaxies?

The program will consist of informal talks and ample free time for discussion and collaborative work. Taking our inspiration from recent meetings such as the ISM Beyond 3D, we hope to provide as much time as possible for open collaboration, with a focus on knowledge-sharing and project development.

We should have the run of the venue, which will be located in the countryside of the Southern Highlands wine region of NSW. The weather in April is usually fresh and warm.