Lofty pillar of square section, tapering at the top and ending pyramidically.
Harvey Lonsdale Elmes,
Of a porch or portico: with eight columns across the front.
Circular opening.
Oeil de boeuf
Small oval window, set horizontally. Also called a bullseye window.
A double curve, bending first one way and then the other. An ogee or ogival arch, especially popular in the 14th century, is pointed at the top. A nodding ogee curves forward from the wall face at the top.
Old English
A style used from c. 1860, in which tile-hanging, tall chimneys, half-timbering and other details of the gabled vernacular architecture of south-east England are picturesquely combined.
Open pediment
A pediment with the centre of the base omitted.
Open string
A sloping member of a staircase covering the ends of the treads and risers and cut into their shape; hence an open-string staircase. Compare closed string.
Opus sectile
Decorative mosaic-like facing.
Opus signinum
Composition flooring of Roman origin.