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O

obelisk:  An Egyptian monument with a tall, tapering shaft of stone with a pyramidal top.
oculus:  A small circular or eye-shaped window.
ogee:  The “s” shaped molding that universally typifies Gothic.   
ogival: Relating to a diagonal rib of a Gothic vault; a pointed arch.
openwork:  Ornamental or structural work, as of embroidery or metal, containing numerous openings, usually in set patterns.
order:  Basic element of classical and Renaissance architecture, comprising the base, column, capital and entablature.
oriel:  A projecting window of an upper storey, or a bay window in an upper story, supported from below by projecting corbels, or brackets of stone or wood. Usually semi-hexagonal or rectangular in plan, oriels first became prevalent early in the 15th century. They were often placed over gateways or entrances to manor houses and public buildings of the late Gothic and Tudor periods. They became popular again during the revivals of these styles in the 19th and early 20th centuries.  A bay window is also called an oriel window, when it projects from an upper story and is supported by corbels.