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OculusA circular or eye shaped window.
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OgiveDiagonal arch which outlines the ribs of the vault of a bay (divided into four or six parts).
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OpeningWindow or opening in a wall.
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Opus sectileInlaid, mosaic decoration (often in marble).
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Opus spicatumSmall tool cut on the diagonal used during the pre-Roman era.
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OvumEgg-shaped ornamental motif.