Oecus is the Latinized form of Greek oikos, used by Vitruvius for the principal hall or salon in a Roman house, which was used occasionally as a triclinium for banquets.

When of great size it became necessary to support its ceiling with columns; thus, according to Vitruvius, the tetrastyle oecus had four columns; in the Corinthian oecus there was a row of columns on each side, virtually therefore dividing the room into nave and aisles, the former being covered over with a barrel vault. The Egyptian oecus had a similar plan, but the aisles were of less height, so that clerestory windows were introduced to light the room, which, as Vitruvius states, presents more the appearance of a basilica than of a triclinium.

Vitruvius distinguishes four types of oecus: