Oaf.  A silly fellow.

Oafish.  Simple.

Oak.  A rich maa, a man of good substance and credit. 
  To sport oak; to shut the outward door of a student’s
  room at college.  An oaken towel; an oaken cudgel.  To
  rub a man down with an oaken towel; to beat him.

Oats.  He has sowed his wild oats; he is staid, or sober,
  having left off his wild tricks.

Oaths.  The favourite oaths of the thieves of the present
  day are, “God strike me blind!” “I wish my bloody eyes
  may drop out if it is not true!” “So help me God!”
  “Bloody end to me!”

Oar.  To put in one’s oar; to intermeddle, or give an
  opinion unasked:  as, To be sure, you must put in your
  oar!

OBSTROPULOUS.  Vulgar misnomer of obstreperous:  as, I
  was going my rounds, and found this here gemman very
  obstropulous, whereof I comprehended him as an
  auspicious parson.

Occupy.  To occupy a woman; to have carnal knowledge
  of her.

ODDFELLOWS.  A convivial society; the introduction to
  the most noble grand, arrayed in royal robes, is well worth
  seeing at the price of becoming a member.

Odds PLUT and her nails.  A Welch oath, frequently
  mentioned in a jocular manner by persons, it is hoped,
  ignorant of its meaning; which is, By God’s blood, and
  the nails with which he was nailed to the cross.

Odd-come-SHORTLYS.  I’ll do it one of these odd-come-shortly’s;
  I will do it some time or another.

Office.  To give the office; to give information, or make
  signs to the officers to take a thief.

Ogles.  Eyes.  Rum ogles; fine eyes.

Oil of barley, or barley broth.  Strong beer.

Oil of gladness.  I will anoint you with the oil of gladness;
  ironically spoken for, I will beat you.

Oil of Stirrup.  A dose the cobler gives his wife whenever
  she is obstropulous.

OI POAAOI (Proofreaders Note:  Greek Letters). 
  (Cambridge.) The many; the multitude;
  who take degrees without being entitled for an honor. 
  All that is required, are three books of Euclid, and as far
  as Quadratic Equation’s in Algebra.  See plucked.

Old.  Ugly.  Cant.

Old dog at it.  Expert, accustomed.

Old hand.  Knowing or expert in any business.

Old Harry.  A composition used by vintners to adulterate
  their wines; also the nick-name for the devil.

Old ding.  See old hat.

Old Mr. Gory.  A piece of gold.

Old nick.  The Devil:  from Neken, the evil spirit of the
  north.