omega Symbol used to denote the ratio of the mass density of the universe to the critical mass density.
opacity A quantity that measures a material's ability to block electromagnetic radiation. Opacity is the opposite to transparency.
open cluster Loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way.
open universe Geometry that the universe would have if the density of matter were less than the critical value. In an open universe there is not enough matter to halt the expansion of the universe. An open universe is infinite in extent.