Answer:
You would expect to find the least number of young O, B stars in the halo of the Milky Way. The halo is the roughly spherical region surrounding the Milky Way, which contains mainly old stars and globular clusters. The disk of the Milky Way, on the other hand, is where most of the young, hot O and B type stars are found, as this is where ongoing star formation is taking place. The halo is made up of older stars that formed early in the history of the Milky Way, so it is not a likely place to find young O and B stars.