We present the detection of molecular oxygen with Odin toward the dense molecular core \rho { Oph A } , which is part of a region of active star formation .
The observed spectral line is the ( N _ { J } = 1 _ { 1 } -1 _ { 0 } ) ground state transition of O _ { 2 } at 119 GHz ( 2.5 mm wavelength ) .
The center of the line is at the LSR velocity of a number of optically thin lines from other species in the region and the O _ { 2 } line also has a very similar , narrow , line width .
Within the 10 ^ { \prime } beam , the line intensity is \int T _ { A } { d } v = 28 mK km s ^ { -1 } , which corresponds to 5 \sigma of the rms noise .
A standard LTE analysis results in an O _ { 2 } abundance of 5 \times 10 ^ { -8 } , with an uncertainty of at least a factor of two .
We show that standard methods , however , do not apply in this case , as the coupling of the Odin beam to the source structure needs to be accounted for .
Preliminary model results indicate O _ { 2 } abundances to be higher by one order of magnitude than suggested by the standard case .
This model predicts the 487 GHz line of O _ { 2 } to be easily detectable by the future Herschel-HIFI facility , but to be out of reach for observations on a shorter time scale with the Odin space observatory .