3O

{{selfref|For the Wikipedia dispute resolution process, see Wikipedia:Third opinion.}}

3O or 3-O may refer to:

• 3J, IATA code for Jubba Airways
• 3O, IATA code for Air Arabia Maroc
• 3o Sector, see Tertiary sector of the economy
• 3-O-sulfation, a phenomenon occurring in Heparan sulfate