[en] This report describes the 1991 implementation of the data acquisition system of the OPAL detector at LEP including the additional services and infrastructure necessary for its correct and reliable operation. The various tasks in this 'on-line' environment are distributed amongst many VME subsystems, workstations and minicomputers which communicate over general purpose local area networks and special purpose buses. The tasks include data acquisition, control, monitoring, calibration and event reconstruction. The modularity of both hardware and software facilitates the upgrading of the system to meet new requirements. (orig.)