c © ESO 2010 Astronomy &Astrophysics

Context. Contrary to theoretical expectation, surprisingly low concentrations of molecular oxygen, O2, have been found in the inter-stellar medium. Telluric absorption makes ground based O2 observations essentially impossible and observations had to be done from space. Millimetre-wave telescopes on space platforms were necessarily small, which resulted in large, several arcminutes wide, beam patterns. Observations of the (NJ = 11−10) ground state transition of O2 with the Odin satellite resulted in a 5σ detection toward the dense core ρOph A. At the frequency of the line, 119 GHz, the Odin telescope has a beam width of 10′, larger than the size of the dense core. Aims. The precise nature of the emitting source and its exact location and extent are therefore unknown. The current investigation is intended to remedy this. Methods. Although the Earth’s atmosphere is entirely opaque to low-lying O2 transitions, it allows ground based observations of the much rarer 16O18O in favourable conditions and at much higher angular resolution with larger telescopes. In addition, ρOph A exhibits both multiple radial velocity systems and considerable velocity gradients. Extensive mapping of the region in the proxy C18O (J = 3−2) line can be expected to help identify the O2 source on the basis of its line shape and Doppler velocity. Line opacities were determined from observations of optically thin 13C18O (J = 3−2). During several observing periods, two C18O intensity maxima in ρOph A were searched for O18O in the (21−01) line at 234 GHz with the 12 m APEX telescope. These positions are associated als