  High resolution X-ray spectra of the Reflection Grating Spectrometer (RGS) on
board the XMM satellite are used to resolve the oxygen K absorption edge. By
combining spectra of low and high extinction sources, the observed absorption
edge can be split in the true interstellar (ISM) extinction and the
instrumental absorption. The detailed ISM edge structure closely follows the
edge structure of neutral oxygen as derived by theoretical R-matrix
calculations. However, the position of the theoretical edge requires a
wavelength shift. In addition the detailed instrumental RGS absorption edge
structure is presented. All results are verified by comparing to a subset of
Chandra LETG-HRC observations.
