| occultation | The passage of an object of large angular size in front of a smaller object, such as the Moon in front of a distant star. |
| Oort comet cloud | The large spherical region around the Sun from which most "new" comets come; a reservoir of objects with aphelia at about 50,000 AU, or extending about a third of the way to the nearest other stars. |
| opacity | Absorbing power; capacity to impede the passage of light. |
| open cluster | A comparatively loose or "open" cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of the Galaxy; sometimes referred to as a galactic cluster. |
| open universe | A model of the universe in which gravity is not strong enough to bring the universe to a halt; it expands forever. In this model the geometry of spacetime is such that if you go in a straight line, you not only can never return to where you started, but even more space opens up than you would expect from Euclidean geometry. |
| optical | in astronomy: relating to the visible-light band of the electromagnetic spectrum. Optical observations are those made mdth visible light. |
| optical double star | Two stars at different distances that are seen nearly lined up in projection so that they appear close together, but that are not really gravitationally associated. |
| orbit | The path of a body that is in revolution about another body or point. |
| oscillation | A periodic motion; in the case of the Sun, a periodic or quasi-periodic expansion and contraction of the whole Sun or some portion of it. |