XO may refer to:
- XO, a grade of cognac, meaning "extra old"
- XO, the IATA code for Spain's LTE International Airways
- XO Communications, a telecom company
- XO sauce, a spicy seafood sauce
- XO Telescope and its related exo-planet survey
- XO-1 laptop, produced by the One Laptop per Child association
- XO-3 laptop, next generation One Laptop per Child laptop
- Crystal oscillator, an electronic circuit that uses the mechanical resonance of a vibrating crystal to create an electrical signal with a precise frequency
- XO, the now-deprecated abbreviation for crystal oscillators on electrical schematic diagrams
- Executive officer in the military
- Executive order (disambiguation), an edict by a member of the executive branch of a government
- Eastern Orthodox Christianity (from the Greek: Χριστιανός Ορθόδοξος, Xristianos Orthodoksos)
- Hugs and kisses, sometimes written as a sequence of XOs
- Xanthine oxidase, an enzyme that catalyzes the oxidation of hypoxanthine to xanthine
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