OPAL (Omni-Purpose Apparatus at LEP) was one of four large detectors on the Large Electron-Positron collider (LEP). It started operation along with the collider in August 1989. Data taking for OPAL ended on 2 November 2000 and the detector was dismantled the following year to make way for construction of the Large Hadron Collider (LHC).

The OPAL detector was about 12 m long, 12m high and 12m wide. Detector components were arranged around the beam pipe, in a layered structure like that of an onion. OPAL's central tracking system consisted of (from the beam pipe out) a silicon microvertex detector, a vertex detector, a jet chamber, and z-chambers. 

The silicon microvertex detector and the vertex chamber worked together to locate decay vertices of short-lived particles, and to improve the momentum resolution. The central jet chamber identified particles from how much ionization they caused, and how far they curved in the magnetic field. These chambers worked well to identify tracks in the plane perpendicular to the beam axis. They were complemented by "z-chambers" at the outside edge of the jet chamber, to provide precise measurements of the perpendicular coordinates of the tracks.

Further out from the beam pipe, OPAL's calorimeter system was divided into electromagnetic calorimeters (to identify electrons), hadron calorimeters (for hadrons) and forward calorimeters placed around, and close to, the beam pipe at the two ends of the detector to catch particles thrown forwards by collisions in LEP. Muon detectors formed the end caps of the detector.

In its first phase of operation from 1989 to 1995, electrons and positrons collided in LEP at 91 GeV. The aim was to produce Z bosons. OPAL accumulated millions of these Z events for high-precision measurements. In LEP's second phase from 1996 to 2000, the collider's collision energy was increased to make pairs of W bosons, and to search for possible new particles and new physics.