Heidelberg Catechism - Lord's Day 37

101. Q . But may we swear an oath in God’s name if we do it reverently?
       A. Yes, when the government demands it, or when necessity requires it, in order to maintain and promote truth and trustworthiness for God’s glory and our neighbor’s good. Such oaths are grounded in God’s Word and were rightly used by the people of God in the Old and New Testaments.

102. Q . May we also swear by saints or other creatures?
       A. No.
A legitimate oath means calling upon God as the only one who knows my heart to witness to my truthfulness and to punish me if I swear falsely. No creature is worthy of such honor.