Context : Contrary to theoretical expectation , surprisingly low concentrations of molecular oxygen , O _ { 2 } , have been found in the interstellar medium by means of orbiting telescopes .

Aims : Observations of the ( N _ { J } = 1 _ { 1 } -1 _ { 0 } ) ground state transition of O _ { 2 } with the Odin satellite resulted in a \stackrel { > } { { } _ { \sim } } 5 \sigma detection toward the dense core \rho { Oph A } .
At the frequency of the line , 119 GHz , the Odin telescope has a beam width of 10 ^ { \prime } , larger than the size of the dense core , so that the precise nature of the emitting source and its exact location and extent are unknown .
The current investigation is intended to remedy this .

Methods : Telluric absorption makes ground based O _ { 2 } 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 .
Although the Earth ’ s atmosphere is entirely opaque to low-lying O _ { 2 } transitions , it allows ground based observations of the much rarer ^ { 16 } { O } ^ { 18 } { O } in favourable conditions and at much higher angular resolution with larger telescopes .
In addition , \rho { Oph A } exhibits both multiple radial velocity systems and considerable velocity gradients .
Extensive mapping of the region in the proxy { C } ^ { 18 } { O } ( J = 3 - 2 ) line can be expected to help identify the O _ { 2 } source on the basis of its line shape and Doppler velocity .
Line opacities were determined from observations of optically thin ^ { 13 } { C } ^ { 18 } { O } ( J = 3 - 2 ) at selected positions .

Results : During several observing periods , two { C } ^ { 18 } { O } intensity maxima in \rho { Oph A } were searched for in the ^ { 16 } { O } ^ { 18 } { O } ( 2 _ { 1 } -0 _ { 1 } ) line at 234 GHz with the 12 m APEX telescope .
These positions are associated with peaks in the mm-continuum emission from dust .
Our observations resulted in an upper limit on the integrated { O } ^ { 18 } { O } intensity of \int T ^ { * } _ { A } { d } \upsilon < 0.01 K km s ^ { -1 } ( 3 \sigma ) into the 26 \stackrel { \prime \prime } { { } _ { \bf \cdot } } 5 beam .

Conclusions : Examining the evidence , which is based primarily on observations in lines of { O } ^ { 18 } { O } and { C } ^ { 18 } { O } , leads us to conclude that the source of observed O _ { 2 } emission is most likely confined to the central regions of the \rho { Oph A core } .
In this limited area , implied O _ { 2 } abundances could thus be higher than previously reported , by up to two orders of magnitude .