The Collaborative International Dictionary of English v.0.48:

Oath \Oath\ ([=o]th), n.; pl. Oaths ([=o][th]z). [OE. othe,
   oth, ath, AS. [=a][eth]; akin to D. eed, OS. [=e][eth], G.
   eid, Icel. ei[eth]r, Sw. ed, Dan. eed, Goth. ai[thorn]s; cf.
   OIr. oeth.]
   1. A solemn affirmation or declaration, made with a reverent
      appeal to God for the truth of what is affirmed. "I have
      an oath in heaven" --Shak.
      [1913 Webster]

            An oath of secrecy for the concealing of those
            [inventions] which we think fit to keep secret.
                                                  --Bacon.
      [1913 Webster]

   2. A solemn affirmation, connected with a sacred object, or
      one regarded as sacred, as the temple, the altar, the
      blood of Abel, the Bible, the Koran, etc.
      [1913 Webster]

   3. (Law) An appeal (in verification of a statement made) to a
      superior sanction, in such a form as exposes the party
      making the appeal to an indictment for perjury if the
      statement be false.
      [1913 Webster]

   4. A careless and blasphemous use of the name of the divine
      Being, or anything divine or sacred, by way of appeal or
      as a profane exclamation or ejaculation; an expression of
      profane swearing. "A terrible oath" --Shak.
      [1913 Webster]

The Devil's Dictionary (1881-1906):

OATH, n.  In law, a solemn appeal to the Deity, made binding upon the
conscience by a penalty for perjury.


Easton's 1897 Bible Dictionary:

Oath
a solemn appeal to God, permitted on fitting occasions (Deut.
6:13; Jer. 4:2), in various forms (Gen. 16:5; 2 Sam. 12:5; Ruth
1:17; Hos. 4:15; Rom. 1:9), and taken in different ways (Gen.
14:22; 24:2; 2 Chr. 6:22). God is represented as taking an oath
(Heb. 6:16-18), so also Christ (Matt. 26:64), and Paul (Rom.
9:1; Gal. 1:20; Phil. 1:8). The precept, "Swear not at all,"
refers probably to ordinary conversation between man and man
(Matt. 5:34,37). But if the words are taken as referring to
oaths, then their intention may have been to show "that the
proper state of Christians is to require no oaths; that when
evil is expelled from among them every yea and nay will be as
decisive as an oath, every promise as binding as a vow."

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "oath":
   Bible oath, assurance, avouch, avouchment, avow, avowal, curse,
   cuss, cuss word, dirty name, dirty word, dysphemism, epithet,
   expletive, extrajudicial oath, faith, foul invective, guarantee,
   guaranty, imprecation, ironclad oath, judicial oath, loyalty oath,
   malediction, naughty word, no-no, oath of allegiance,
   oath of office, obscenity, official oath, parole, pledge, plight,
   profane oath, profanity, promise, solemn declaration, solemn oath,
   swear, swearword, sworn statement, test oath, troth, vow, warrant,
   warranty, word, word of honor