Each commissioner before he or she enters upon the duties of his or her office shall take and subscribe an oath or affirmation that he or she will faithfully and impartially discharge the duties of his or her office to the best of his or her ability. This oath, or affirmation, shall be administered and certified by an officer of the county in which the district is situated, who is authorized to administer oaths, without charge therefor. The oath or affirmation shall be filed with the county auditor.