  Interstellar dust permeates our Galaxy and plays an important role in many
physical processes in the diffuse and dense regions of the interstellar medium.
High-resolution X-ray spectroscopy, coupled with modelling based on laboratory
dust measurements, provides a unique probe to investigate the interstellar dust
properties along our line of sight towards Galactic X-ray sources. Here, we
focus on the oxygen content of the interstellar medium through its absorption
features in the X-ray spectra. To model the dust features, we perform a
laboratory experiment using the electron microscope facility located at the
University of Cadiz in Spain, where we acquire new laboratory data in the
oxygen K-edge. We study 18 dust samples of silicates and oxides with different
chemical compositions. The laboratory measurements are adopted for our
astronomical data analysis. We carry out a case study on the X-ray spectrum of
the bright low-mass X-ray binary Cygnus X-2, observed by XMM-Newton. We
determine different temperature phases of the ISM, and parameterize oxygen in
both gas (neutral and ionised) and dust form. We find Solar abundances of
oxygen along the line of sight towards the source. Due to both the relatively
low depletion of oxygen into dust form and the shape of the oxygen cross
section profiles, it is challenging to determine the precise chemistry of
interstellar dust. However, silicates provide an acceptable fit. Finally, we
discuss the systematic discrepancies in the atomic (gaseous phase) data of the
oxygen edge spectral region using different X-ray atomic databases, and also
consider future prospects for studying the ISM with the Arcus concept mission.
