65 ILCS 5/6-4-9

Before entering upon the duties of their respective offices all officers, whether elected or appointed, shall take and subscribe the following oath: I do solemnly swear 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 .... according to the best of my ability. This oath, so subscribed, shall be filed in the office of the city or village clerk.