Nearly 6-7% of massive OB stars possess a magnetic field. We review recent investigations of these fields. Special attention is given to the studies of magnetic properties of high-mass X-ray binaries. A model involving the interaction of magnetized stellar wind with a neutron star magnetosphere is considered to characterize the behavior of supergiant fast X-ray transients. The presence of magnetic fields in Be stars is also discussed. We argue that the observed fraction of magnetic stars is determined by physical conditions at the pre-main-sequence stages of stellar evolution. We also consider the recent finding of fast line-profile variations on a timescale of minutes and even a fraction of a minute in spectra of OBA stars. The existence of compact regions with local magnetic fields on the surface of massive OB stars and their impact on fast line-profile variations is also discussed.