The Observation Software (OS) is the top level control software of astronomical instruments which is managing the
actions during exposures and calibrations carried out at ESO (at various sites VLT, VLTI, La Silla, VISTA). The
software framework Base Observation Software Stub (BOSS) provides the foundation of the OS, in use for a decade.
BOSS contains 26000 lines of C++ code and covers the functionalities of a simple OS (configuration, synchronization of
the subsystems, state alignment, exposure and image file handling). The need for ever increasing precision and speed
imposes a consequent increase in complexity on the astronomical instrument control software. Thus makes the OS a
critical component in the instrument design. This is reflected by the size of the BOSS applications varying between 0-12000 lines including additional scheduler mechanism, calculation of optical phenomena, online calibrations etc. This
article focuses on the progress of OS and BOSS, and their functionality over time.