OB or Ob may refer to:

• OB star, in astronomy, a hot, massive star of spectral types O or B
• Odense Boldklub, a Danish football club
• Organizational behaviour, an academic field of study
• Oriental Brewery, a South Korean brewery, which produces the OB lagers
• Outside broadcasting, a television broadcasting from a mobile television studio, such as a van
• Outward Bound, an international outdoor education organization
• Astrakhan Airlines (IATA code)
• Boliviana de Aviación (IATA code)
• Halfpenny (British pre-decimal coin), from "ob", archaic abbreviation (see, e.g., Shakespeare's Henry IV, Part One, II.iv.539)
• Object Manager (Windows), a Windows NT subsystem
• The fictional currency of the Gands in Eric Frank Russell's The Great Explosion
• Orphan Black, a popular Canadian science fiction television show.
• Recipient of the Order of Burma

The Supreme Commander (; acronym: ÖB) is the highest ranked professional military officer in the Swedish Armed Forces, and is by NATO terminology the Swedish chief of defence equivalent. The Supreme Commander is the agency head of the Swedish Armed Forces and formally reports to the Government of Sweden, though normally through the Minister for Defence. The primary responsibilities and duties of the Supreme Commander (and the charter for the Armed Forces) are prescribed in an ordinance issued by the Government.

Østerbros Boldklub (also known as ØB) was a Danish sports club based in the Østerbro area of Copenhagen, active from 1894 to 1998. The club is best known for its amateur football team. It was one of the oldest Danish football clubs, and played 10 seasons in the top-flight Danish football championship from 1928 to 1950. ØB was a founding member of the Sports Confederation of Denmark (1896) and the Copenhagen Football Association (1903). The football club predominantly played its games at the Fælledparken, a common shared by the smaller Copenhagen clubs.