omentum
  Folds of the peritoneum (membrane lining the abdominal cavity) that
  connect the stomach with other abdominal organs.

ophthalmia neonatorum (infantile purulent conjunctivitis)
  Various forms of conjunctivitis in newborns, usually contracted during
  birth from passage through the infected birth canal of the mother.

orchitis
  Inflammation of the testes, often the result of mumps or other
  infection, trauma, or metastasis.

organdy (organdie)
  Stiff transparent fabric of cotton or silk, used for trim, curtains, and
  light apparel.

Origanum
  Marjoram.  Genus of mint-like plants (Origanum).  The sweet marjoram (O. 
  Majorana) is aromatic and fragrant, and used in cooking.  The wild
  marjoram of Europe and America (O. vulgare) is less fragrant.

orris
  Several species of iris with a fragrant rootstock, especially Iris
  germanica, used in perfumes and cosmetics.

panada
  Paste or gruel of bread crumbs, toast, or flour combined with milk,
  stock, or water; used for soups or thickening sauces.

Paralysis Agitans (Parkinson’s disease, shaking palsy)
  Progressive nervous disease causing destruction of brain cells that
  produce dopamine, muscular tremor, slowing of movement, partial facial
  paralysis, peculiarity of gait and posture, and weakness.

paregoric
  A camphorated tincture of opium, taken internally for the relief of
  diarrhea and intestinal pain

Paris green
  Poisonous emerald-green powder, C4H6As6Cu4O16, used as a pigment,
  insecticide, and wood preservative.

pedicle (pedicel)
  Small stalk or stalk-like structure, especially one supporting or
  connecting an organ or other body part.  Slender foot-like part, as at
  the base of a tumor.

pell mell
  Jumbled, confused manner; helter-skelter; frantic disorderly haste;
  headlong: 

pemphigus
  Several acute or chronic skin diseases characterized by groups of
  itching blisters.

pennyroyal
  Eurasian mint (Mentha pulegium) with small lilac-blue flowers that yield
  an aromatic oil.  Aromatic plant (Hedeoma pulegioides) of eastern North
  America, having purple-blue flowers that yields an oil used as an insect
  repellent

peptonize
  Convert protein into a peptone (water-soluble protein derivative
  produced by partial hydrolysis of a protein by an acid or enzyme ). 
  Dissolve (food) by means of a proteolytic enzyme.

pernicious anemia (Addison’s anemia, malignant anemia.)
  Severe anemia in older adults, caused by failure absorb vitamin B12;
  causes abnormally large red blood cells, gastrointestinal disturbances,
  and lesions of the spinal cord.

pharyngitis
  Inflammation of the pharynx.

phenacetine (phenacetin)
  White, crystalline compound, C10H13O2N, used as an antipyretic.