O&D stands for "Origin and Destination." O&D passengers are the number of passengers flying between two specific points, such as ORD
O&D passengers, in this instance, are the number of passengers whose trips began at ORD
and ended at LAX
or vice versa.
O&D numbers do not typically include passengers flying ORD
but are connecting at one or both of those points to their final destination. For example, a PIA-LAX
passenger connecting over ORD
should be counted only in the PIA-LAX
O&D, and not counted in the ORD
O&D. There are some exceptions to that example that have to do with how passengers are ticketed...