Context : The Herschel Oxygen Project ( HOP ) is an Open Time Key Program , awarded 140 hours of observing time to search for molecular oxygen ( O _ { 2 } ) in a number of interstellar sources .
To date O _ { 2 } has definitely been detected in only two sources , namely \rho Oph A and Orion , reflecting the extremely low abundance of O _ { 2 } in the interstellar medium .

Aims : One of the sources in the HOP program is the +50 km s ^ { -1 } Cloud in the Sgr A Complex in the centre of the Milky Way .
Its environment is unique in the Galaxy and this property is investigated to see if it is conducive to the presence of O _ { 2 } .

Methods : The Herschel Heterodyne Instrument for the Far Infrared ( HIFI ) is used to search for the 487 and 774 GHz emission lines of O _ { 2 } .

Results : No O _ { 2 } emission is detected towards the Sgr A +50 km s ^ { -1 } Cloud , but a number of strong emission lines of methanol ( CH _ { 3 } OH ) and absorption lines of chloronium ( H _ { 2 } Cl ^ { + } ) are observed .

Conclusions : A 3 \sigma upper limit for the fractional abundance ratio of [ O _ { 2 } ] / [ H _ { 2 } ] in the Sgr A +50 km s ^ { -1 } Cloud is found to be X ( O _ { 2 } ) \leq 5 \times 10 ^ { -8 } .
However , since we can find no other realistic molecular candidate than O _ { 2 } itself , we very tentatively suggest that two weak absorption lines at 487.261 and 487.302 GHz may be caused by the 487 GHz line of O _ { 2 } in two foreground spiral arm clouds .
By considering that the absorption may only be apparent , the estimated upper limit to the O _ { 2 } abundance of \leq ( 10 - 20 ) \times 10 ^ { -6 } in these foreground clouds is very high , as opposed to the upper limit in the Sgr A +50 km s ^ { -1 } Cloud itself , but similar to what has been reached in recent chemical shock models for Orion .
This abundance limit was determined also using Odin non-detection limits , and assumes that O _ { 2 } fills the beam .
If the absorption is due to a differential Herschel OFF-ON emission , the O _ { 2 } fractional abundance may be of the order of \approx ( 5 - 10 ) \times 10 ^ { -6 } .
With the assumption of pure absorption by foreground clouds , the unreasonably high abundance of ( 1.4 - 2.8 ) \times 10 ^ { -4 } was obtained .
The rotation temperatures for CH _ { 3 } OH- A and CH _ { 3 } OH- E lines in the +50 km s ^ { -1 } Cloud are found to be \approx 64 and 79 K , respectively , and the fractional abundance of CH _ { 3 } OH is approximately 5 \times 10 ^ { -7 } .