O & D is short for Origin and Destination.
This is a statement in regards to the number of passengers, from point A to B, no matter how they get there. That means that all connections, nonstops etc.
In the United States, to find exact O&Ds its a bit difficult, however the DOT, publishes free data that one can analyze to find out the numbers for a particular O&D pair.
You can find this data at
However it is quite difficult to analyze this data without knowledge of Access and Excel.
And this does not include any International City pairs. or Domestic to International.
If you are looking for a particular O&D, let me know.
Hope that helps.