Boyd’s Bible Dictionary:

Appeals to God to attest the truth of an assertion in early use
(Gen. 21:23; 26:323Now therefore swear unto me here by God that thou wilt not deal falsely with me, nor with my son, nor with my son's son: but according to the kindness that I have done unto thee, thou shalt do unto me, and to the land wherein thou hast sojourned. (Genesis 21:23)
3Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father; (Genesis 26:3)
; Heb. 6:1616For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. (Hebrews 6:16)). Regulated in Ex. 20:77Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain. (Exodus 20:7); Lev. 19:1212And ye shall not swear by my name falsely, neither shalt thou profane the name of thy God: I am the Lord. (Leviticus 19:12). Forms: lifting hands (Gen. 14:2222And Abram said to the king of Sodom, I have lift up mine hand unto the Lord, the most high God, the possessor of heaven and earth, (Genesis 14:22)); placing hand under thigh (Gen. 24:22And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh: (Genesis 24:2)); before the altar (1 Kings 8:3131If any man trespass against his neighbor, and an oath be laid upon him to cause him to swear, and the oath come before thine altar in this house: (1 Kings 8:31)); laying hand on the law.

Concise Bible Dictionary:

In the common intercourse of life there should be no oaths, the simple “yea” and “nay” should be enough, “swear not at all” (Matt. 5:34-3734But I say unto you, Swear not at all; neither by heaven; for it is God's throne: 35Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. 36Neither shalt thou swear by thy head, because thou canst not make one hair white or black. 37But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil. (Matthew 5:34‑37); James 5:1212But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation. (James 5:12)): the context of these passages shows that they do not refer to judicial oaths (compare also Heb. 6:13,1613For when God made promise to Abraham, because he could swear by no greater, he sware by himself, (Hebrews 6:13)
16For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. (Hebrews 6:16)
; Heb. 7:2121(For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:) (Hebrews 7:21); Rev. 10:66And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer: (Revelation 10:6)).