You'll need to track down the name of the outdoor air node entering any central air system diagram. To do this:

1. While viewing a central air system diagram in the HVAC Systems tab, click on the dot that represents the outdoor air node.
2. See the name for the outdoor air node appear on the right side of OpenStudio.

You can also update node names on the right after clicking on them in HVAC diagrams, in case you want to apply a naming scheme that makes sense to you instead of the default OpenStudio "Node X" numbering.