MMO

MMO may refer to:
Organizations
Marine Management Organisation, a non-departmental public body of the UK Government
Music Minus One, a music industry company

Science, technology, and biology
Marine Mammal Observer, a professional in environmental consulting
Methane monooxygenase, an enzyme
Mixed metal oxide, a type of electrode
Matyas-Meyer-Oseas (cryptography), a construction to build a hash function from a block cipher
Mach Maximum Operating (MMO), a speed limit shown on a machmeter
Mercury Magnetospheric Orbiter, a key component of the ESA-led Mercury mapper BepiColombo
Mini-Mag Orion, a proposed type of spacecraft propulsion

Other uses
Massively multiplayer online game, a multiplayer video game which is capable of supporting hundreds or thousands of players simultaneously
Means, motive, and opportunity, three aspects of a crime (U.S.)
Melton Mowbray railway station, National Rail station code MMO (England)
Minimum Municipal Obligation, minimum contribution to a pension plan