55 ILCS 5/2-4001

Each member of the board of county commissioners shall be commissioned by the governor, and shall, before entering upon the duties of his office, take and subscribe the following oath, which shall be filed in the office of the county clerk: I do solemnly swear (or affirm, as the case may be), that I will support the constitution of the United States, and the constitution of the state of Illinois, and that I will faithfully discharge the duties of the office of county commissioner of .... county, according to the best of my ability.