"In the sixth book I will include commandments for which a man becomes responsible if he forbids things to himself, as by oaths or vows. And I have called this book the Book of Separation."

30. Oaths - Shevuos

a) Types of oaths

1) False oaths: It is forbidden to swear falsely, as it says "And you shall not swear falsely in My name".¹ One who swears an oath about a past or future event and is wrong has sworn a false oath, provided the event is something that it is possible (and not forbidden) for him to do or know about. This type of oath is called an "oath of expression" (shevuas bitui), as it says "Or if a person swears, expressing with his lips, for evil or for good".^2,a

2) Vain oaths: It is forbidden to swear vainly, as it says "You shall not take the Name of Ha-Shem your G-d in vain".³ There are four types of vain oaths: swearing that a well-known thing is or is not so; swearing not to observe a commandment; and swearing to do something that it is not possible to do. Swearing to observe a commandment is not forbidden, but such an oath is not valid.^b

3) Oaths about deposits: If anyone has someone else's real, movable property in his posession by deposit, loan, theft, extortion, or loss and it is demanded of him he is forbidden to deny it, as it says "You shall not deceive"⁴; and there is a further prohibition against swearing falsely about it, as it says "And you shall not lie one to another".⁴ If one swears such a false oath (about something that he would have been required to pay for had he admitted the truth) he must pay 5/4 of the amount and (when the Temple exists) must bring a sin-offering [as it says "If a person sins... and deceives another regarding deposited property or a pledge or theft, or extorts from another or finds lost property and denies it, and swears falsely... he must return what he stole or what he extorted or what was deposited with him or the lost thing that he found or whatever he swore falsely about, and pay it in full and add a fifth to it..."].^5,c

4) Oaths about testimony: If witnesses are asked by someone to testify about his claim to real, movable property, and their testimony alone would enable him to collect his claim, but they falsely swear (and deny in court) that they do not know anything about which they could testify for him, they must (when the Temple exists) bring a sin-offering, as it says "And if a person sins and hears a request to swear and he is a witness [or saw or knew something], if he does not tell he bears his sin."^6,d

5) Oaths administered by judges: If a monetary claim is made against someone and he denies part of it (rabbinically, even all of it), or if a single witness testifies against him, or if he is a depositor and maintains that there was a loss for which he is not responsible, he must swear in court, as it says "There shall be an oath of Ha-Shem between them".⁷ Many other types of oaths regarding claims were instituted by the sages. If one swears any such oath falsely he has violated the prohibition regarding "oaths about deposits".^e

b) Laws of oaths

An oath must be phrased so as to indicate that swearing is intended; there is no punishment unless a Name or attribute of G-d is mentioned. It must be expressed orally and the speaker's intent must agree with what is said. An oath can be administered by others and one can accept it by agreeing to it (and to its intent), but one cannot swear by including himself in an oath sworn by another or extend the scope of an oath to new events or things.^f If the swearer is not aware of the facts or is under duress or is exaggerating he is not culpable.^g

One can be released from an oath by a scholar or by any three men, provided he regrets the oath on grounds which did not exist or which he did not appreciate at the time of the oath; this is a tradition received from Moses. However, this should be done only in cases of great need.^h

If one is required to swear in court he is commanded to swear in G-d's Name, as it says "And in His Name you shall swear".^8,i Swearing falsely or mentioning G-d's Name unnecessarily is a desecration of the Name, as it says "[And you shall not swear falsely by My Name] and desecrate the Name of your G-d; I am Ha-Shem"¹; and it says "For Ha-Shem will not cleanse [one who swears by His Name in vain]".^3,j

Sources:
1. Lev. 19:12	a. 1:1-3; 5:16-17
2. Lev. 5:3	b. 1:4-7; 5:16 (and see 11:3)
3. Ex. 20:7, Deut. 5:11	c. 1:8-9; 7:2-4; see Arachin 4:5. (The "fifth" is a fifth of the total payment, i.e. a quarter of the principal.)
4. Lev. 19:11	d. 1:12; 9:1
5. Lev. 5:21-24	e. 11:4-7,20
6. Lev. 5:1	f. 2:5,4,10-11,1,15,8-9
7. Ex. 22:10	g. 3:1,5,6
8. Deut. 10:20	h. 6:1-2; 12:12
	i. 11:1 (and see Avodah Zarah 5:10)
	j. 12:1,11