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
operameterAn instrument or machine for measuring work done, especially the number of rotations by a machine or wheel in manufacturing cloth.
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).
opere citatoIn the work (already) cited or quoted. Used, typically in footnotes and endnotes, to cite in an abbreviated form a source that has been cited previously; frequently abbreviated as op. cit.
optimeA student who graduates with second class ("senior optime") or third class ("junior optime") honours in mathematics, or (loosely) in any other subject.
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.
-orCreates an agent noun, often from a verb, indicating a person or object (often machines or parts of them) that do the verb or part of speech with which they are formed.
oransA figure in an attitude of prayer, usually standing, with the elbows close to the sides of the body and with the hands outstretched sideways, palms up.
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.
orbiculaA dorsal sclerite over a hymenopteran arolium; a hard exoskeletal feature with species-specific shape that overhangs and protects the adhesive tip of such an insect's leg and around which the claw(s) curl. Abbreviation "or" or "rb".