On-glaze — Decoration executed on already fired glaze with enamels. Because on-glaze enamels can be fired at lower temperatures, a more varied range of colours is available than with under-glaze decoration.
Opaque glazes — Coloured glazes that cover the tile without allowing the body to show through. They are made by adding coloured oxides and other opacifiers to transparent glazes.
OPUS — Latin term which means "work". It is used in conjunction with other Latin terms to describe different ways of laying tiles to create different patterns and effects.
ORNATO — Terrazzo floor with various patterns and decorations.
ORSO — A piece of sandstone fixed on a wood stick used to polish terrazzo floors.
OXIDE — Chemical dye in powder form used to colour mosaic cement and adhesives.
