onus probandiThe burden of proof, the onus; the duty of a party in a legal proceeding to prove an assertion of fact; it includes both the burden of production and the burden of persuasion.
onychathe operculum of kinds of strombus or muricid, smoked as an ingredient in the Mosaic incense and pre-modern medicine
operandumA lever, key or device installed in an operant chamber, upon which the "subject" exerts its responses. Sometimes referred to as a manipulandum. In the prototypical operant conditioning experiment, a rat (the subject) presses upon a lever (the operandum) which triggers the delivery of food (the reward or reinforcer).
optimeA student who graduates with second class ("senior optime") or third class ("junior optime") honours in mathematics, or (loosely) in any other subject.
opus anglicanumFine medieval English needlework, often using gold and silver threads on a rich velvet or linen ground.
Opus DeiThe Prelature of the Holy Cross and Opus Dei, a Roman Catholic organization composed of a prelate, secular clergy and lay people, whose mission is to spread the Catholic teaching that everyone is called to become a saint; its lay members, men and women, engage in the affairs of the world and seek to direct them "according to God's will".
opus latericiumAn Ancient Roman construction technique in which coursed brickwork is used to face a core of opus caementicium.
opus quadratumAn ancient Roman construction technique in which squared blocks of stone of the same height were set in parallel courses, most often without the use of mortar.
opus reticulatumA facing used for concrete walls in Ancient Roman architecture, with small pyramidal tuff embedded in a concrete core.
opus sectileA form of pietra dura, popular in the ancient and medieval western world, in which materials are cut and inlaid into walls and floors to create images or patterns.
opus vermiculatumA method of laying mosaic tesserae to emphasize an outline around a subject.
ora serrataThe serrated junction between the retina and the ciliary body, marking the transition from the simple non-photosensitive area of the ciliary body to the complex, multilayered photosensitive region of the cilia.
OrestisAn ancient district of Macedonia, in modern Greece
orexisThe affective and conative character of mental activity as contrasted with its cognitive aspect; the appetitive aspect of an act; desire, appetite.
osculatoryA religious tablet, usually carrying a representation of Christ or the Virgin Mary, which is kissed by the priest during the Mass ("kiss of peace"); it is then passed to others at the mass who also kiss it.
osculumOne of the suckers on the head of a tapeworm.
PaimonA follower of Lucifer, a demon king, the 9th king of Hell, ruler of the western hells.
PalaepolisA former city in southwest Italy, now part of Naples.
palaestraA public area in ancient Greece and Rome dedicated to the teaching and practice of wrestling and other sports; a wrestling school, a gymnasium.