The Chicago Department of Aviation (CDA) compiles monthly statistics, including flight operations, passenger totals and cargo tonnage. The data is provided by the airlines and summarized by the CDA. In 2014, O'Hare was the busiest airport in the world in terms of number of annual aircraft operations. In 2010, O'Hare the third busiest in terms of number of annual passengers and was the18th busiest for air cargo.
Please note that some of the links contain two pages of data, one for O'Hare and one for Midway.
Monthly reports starting from January 2000: