Further development of gallium nitride (GaN) based optoelectronic devices requires in-depth understanding of the defects present in GaN grown on a sapphire substrate. In this work, we present three dimensional secondary ion mass spectrometry (SIMS) detection of oxygen. Distribution of these impurities is not homogeneous and the vast majority of oxygen atoms are agglomerated along pillar-shaped structures. Defect-selective etching and scanning electron microscopy imaging complement SIMS results and reveal that oxygen is predominantly present along the cores of screw and mixed dislocations, which proves their high tendency to be decorated by oxygen. A negligible amount of oxygen can be found within the bulk of the material and along the edge dislocations.