OG, O.G., or Og may refer to:

• Og, an ancient Amorite king of Bashan
• Objective glass in optics, especially refractive telescopes, the objective lens
• Obstetrics and gynaecology, the two medical specialties dealing with the female reproductive organs
• Olive green, a common colour in military use
• One-Two-GO Airlines (IATA airline code: OG)
• Orange G, a counterstain used in the Pap stain
• Original gangster (disambiguation), various meanings derived from a slang term for a veteran gang member
• Original Gangsters (gang), a Swedish gang
• Original gravity, used in calculating the alcohol content of a drink
• River Og, a river in Wiltshire, England
• og, a namespace identifier typically tied in web pages to the Facebook Open Graph protocol

Og (, ʿog ; , ^cogh ) according to The Torah, was an Amorite king of Bashan who, along with his army, was slain by Moses and his men at the battle of Edrei. In Arabic literature he is referred to as ‘Uj ibn Anaq (‘Ûj ibn ‘Anâq عوج بن عنق).

Östgöta nation or ÖG, as it is called informally, is a student society and one of thirteen nations at Uppsala University. Though Östgöta nation had existed in various forms previously, the nation's constitution was drawn up on 8 November 1646 and that is now considered its official date of creation.