Context : Oxygen sequence Wolf-Rayet ( WO ) stars represent a very rare stage in the evolution of massive stars .
Their spectra show strong emission lines of helium-burning products , in particular highly ionized carbon and oxygen .
The properties of WO stars can be used to provide unique constraints on the ( post- ) helium burning evolution of massive stars , as well as their remaining lifetime and the expected properties of their supernovae .

Aims : We aim to homogeneously analyse the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state .

Methods : We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and deduce the atmospheric parameters .
We calculate dedicated evolutionary models to determine the evolutionary state of the stars .

Results : The WO stars have extremely high temperatures that range from 150 kK to 210 kK , and very low surface helium mass fractions that range from 44 % down to 14 % .
Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of M _ { \mathrm { He,ini } } = 15 - 25 M _ { \odot } that exhibit a fairly strong ( on the order of a few times 10 ^ { -5 } M _ { \odot } \mathrm { yr } ^ { -1 } ) , homogeneous ( f _ { \mathrm { c } } > 0.3 ) stellar wind .

Conclusions : WO stars represent the final evolutionary stage of stars with estimated initial masses of M _ { \mathrm { ini } } = 40 - 60 M _ { \odot } .
They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years .