Context : The ortho-to-para ratio ( OPR ) of H _ { 2 } O is thought to be sensitive to the temperature of water formation .
The OPR of H _ { 2 } O is thus useful to study the formation mechanism of water .

Aims : We investigate the OPR of water in the Orion PDR ( Photon-dominated region ) , at the Orion Bar and Orion S positions , using data from Herschel /HIFI .

Methods : We detect the ground-state lines of ortho- and para-H _ { 2 } ^ { 18 } O in the Orion Bar and Orion S and we estimate the column densities using LTE and non-LTE methods .

Results : Based on our calculations , the ortho-to-para ratio ( OPR ) in the Orion Bar is 0.1 - 0.5 , which is unexpectedly low given the gas temperature of \sim 85 K , and also lower than the values measured for other interstellar clouds and protoplanetary disks .
Toward Orion S , our OPR estimate is below 2 .

Conclusions : This low OPR at 2 positions in the Orion PDR is inconsistent with gas phase formation and with thermal evaporation from dust grains , but it may be explained by photodesorption .