Ordo (Latin "order, rank, class") may refer to:

• A musical phrase constructed from one or more statements of a rhythmic mode pattern and ending in a rest
• Big O notation in calculation of algorithm computational complexity
• Ordo (palace) (Orda or Horde), is a nomadic palace for the Mongol aristocrats and the Turkic rulers
• Order (biology), in the taxonomy of organisms
• Ordo Recitandi: The Ordo gives complete details of the celebration of the Eucharist and the Liturgy of the Hours, beginning with the first Sunday of Advent
• The Inquisition from Warhammer 40,000 has three main ordines:The Ordo Malleus,The Ordo Hereticus and The Ordo Xenos
• Monastic order
• Ordo Templi Orientis, an organization dedicated to the religious philosophy of Thelema
• The scholarly economic/political science journal ORDO, The Annual Review of Economic and Social Order
• Canderous Ordo, a fictional character in the Star Wars video games Star Wars: Knights of the Old Republic and Star Wars: Knights of the Old Republic II: The Sith Lords
• A fictional encryption program from the book Cryptonomicon, by Neal Stephenson
• Novus ordo seclorum which appears on the reverse of the Great Seal of the United States
• Ordo Missae, the order (regulation) of the Eucharist in the Roman Rite of the Catholic Church

ORDO — Jahrbuch für die Ordnung von Wirtschaft und Gesellschaft (English: The Ordo Yearbook of Economic and Social Order, most commonly referred to as Ordo Yearbook, or simply as ORDO) is a peer-reviewed academic journal established in 1948 by the German economists Walter Eucken and Franz Böhm. The journal focuses on the economic and political institutions governing modern society.