Wikipedia
OKB is a transliteration of the Russian initials of "Опытное конструкторское бюро" – Opytnoye Konstruktorskoye Buro, meaning Experimental Design Bureau. During the Soviet era, OKBs were closed institutions working on design and prototyping of advanced technology, usually for military applications.
A bureau was officially identified by a number, and often semi-officially by the name of the lead designer – for example, OKB-51 was led by Pavel Sukhoi, and eventually became known as OKB of Sukhoi. Successful and famous bureaus often retained this name even after the death or replacement of their designer.
These relatively small state-run organisations were not intended for mass production of aircraft, rockets, or other vehicles or equipment they designed. However they usually had the facilities and resources to construct prototypes. Designs accepted by the state were then assigned to factories for mass production.
After the collapse of the Soviet Union, many OKBs became Scientific Production Organizations (Научно-производственное объединение) abbreviated to NPO. There were some attempts to merge them in the 1990s, and there were widespread amalgamations in 2001–2006 to create "national champions", such as Almaz-Antey to consolidate SAM development.