Eastern Orthodoxy (Christianity), from the Greek: Xristianos Orthodoksos (Χριστιανός Ορθόδοξος)

Tic-tac-toe, a game using X's and O's

Turner Syndrome

Crystal oscillator

SpaceShipOne flight 15P

XO (album), an album by Elliott Smith

X (Xbox show)

Badtz-Maru, often represented "XO", a character from Sanrio's Hello Kitty series

XML object

The sequence of letters denoting hugs and kisses used to express affection in writing (e.g., XOXO)

Executive officer (military and not only)

XO sauce

XO Communications, a telecom company

Turner Syndrome or X0 sex-determination system (genetics)

XO Telescope and its related exoplanet survey

Executive order, an edict issued by a member of the executive branch of a government.

XO-1 laptop (previously known as the $100 Laptop or Children's Machine), produced by the One Laptop per Child association

"XO", a song from From Under the Cork Tree, an album by Fall Out Boy

Spain's LTE International Airways, IATA code for LTE International Airways

XO (Extra Old), an unofficial grade used in marketing Cognac and other brandies, where the youngest brandy is stored at least six years in cask