Longest Oath Known to Legal History,

or at least to me, uncovered by Marc Abrahams of the Annals of Improbable Research in Kenneth R.H. MacKenzie's Burmah and the Burmese (1853) (short video here). It apparently began (paragraph breaks added):

If I speak not the truth, may it be through the influence of the laws of demerit, viz.: passion, anger, folly, pride, false opinion, hardheartedness, and scepticism; so that when I and my relations are on land, land animals, as tigers, elephants, buffaloes, poisonous serpents, scorpions, &c., shall seize, crush, and bite us, so that we shall certainly die.

Let calamities occasioned by fire, water, rulers, thieves and enemies, oppress and destroy us, till we come to utter destruction. Let me be subject to all the calamities that are within the body, and all that are without the body. May we be seized with madness, dumbness, blindness, leprosy and hydrophobia. May we be struck with thunderbolts, and lightning, and come to sudden death.

In the midst of not speaking truth, may I be taken with vomiting black clotted blood, and suddenly die before the assembled people. When I am going by water, may the water Nats assault me, the boat be upset, and the property lost; and may alligators, porpoises, sharks, or other sea monsters, seize and crush me to death; and when I change worlds, may I not arrive among men or Nats, but suffer unmixed punishment and regret in the utmost wretchedness among the four states of punishment, Hell, Prita, Beasts and Athurakai.

There's more.