James 5:12 But above all, my brothers, do not swear, either by heaven or by earth or by any other oath, but let your “yes” be yes and your “no” be no, so that you may not fall under condemnation. 

In the last Chapter of James, the Holy Spirit tells us not to be full of ourselves, but to simply and concisely state the truth of God. Adding a lot of excess commentary, may bring about our own condemnation. Swearing by any oaths takes focus off the pure action of Jesus!  Jesus said: 

Matthew 5:37 Let what you say be simply ‘Yes’ or ‘No’; anything more than this comes from evil.