Catechism of the Catholic Church §2154 — excerpt
Following St. Paul, The tradition of the Church has understood Jesus' words as not excluding oaths made for grave and right reasons (for example, in court).
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: CIC, can. 1199 # 1