Where can I see what the design specification outdoor air is doing in openstudio? I see this in the library and attached to my spaces, but what function does it perform? Can I create my own definitions of this not using library values?
A picture is probably the most helpful. 
This is from the facility tab. You can click on the design oa spec and edit the properties on the right.
If you have ZoneHVACEquipment like a PTAC, or a AirLoopHVAC based system like Packaged Rooftop Unit from the OS templates, and those system's oa rates are autosized, then they will look back to these design oa spec objects to figure out how much oa to deliver. OpenStudio will have these systems autosized by default, so the inputs you define here will specify how much mechanical ventilation you receive.
Behind the scenes these design oa spec objects get attached to the EnergyPlus ZoneSizing object associated with spaces.
Ventilation can get complicated, especially if there are multiple pieces of equipment serving the same zone. I am developing some documentation for OpenStudio to hopefully clear it up, but this understanding should get you going.
Regarding the last question in this post, "How do I create my own definitions". I have also found this confusing. It doesn't seem that you can use the "+" "X2" and "X" buttons to add, copy and delete OA definitions (Someone please correct me if I'm mistaken here).
So what I typically do is: load any space type from a library, then select its OA definition, go to edit on upper left and rename it and edit it for your specific needs. You can then drag this from "my model" tab to whatever space type you want to use it in.
I agree that this is confusing. Kind of an orphan in the OpenStudio interface. Seems like Design Specification Outdoor Air could be added to the "Loads" section along with People Definition, Lights Definition, etc. That would allow one to copy, add, edit outdoor air specifications in order to then select them in the Space Type section.
Seems like the same issue applies to the Space Infiltration Flow Rates (SpaceInfiltration:DesignFlowRate) objects and the Space Infiltration Effective Leakage Areas objects.
And these are now in the Space Types section rather than Facilities.
I have also had issues finding where to create OA and Infiltration Rates, requiring me to follow the exact procedure @Hayes Zirnhelt described. Is there any development in the works to improve this workflow and provide a more clear interface for these components?
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