I report the results of the first all-sky survey of Hα emission in the spectra of O-type binaries. The survey includes 26 systems, of which 10 have emission that extends clearly above the continuum. This is the first report of emission for four of these. An additional three systems show small distortions in the Hα profile that may result from weak emission. I compare the distribution of emission systems in H-R diagrams for both binary and single stars, using a survey of single O-type stars done by Conti (1974). Emission in main-sequence systems is extremely rare and is completely absent in my sample of binary stars. Among binary stars, 78% of the systems containing giants show some emission, while no single giants in Conti's sample do. In the case of supergiants, 78% of single stars show emission, while all supergiant binaries show strong emission. Hα emission may come from a variety sources, but the fact that binaries have a higher incidence and strength of emission in post-main-sequence stages may indicate that wind interactions are a common source of emission in massive binaries. To ascertain whether or not colliding winds have been observed, it will be necessary to study the Hα line profile throughout several orbits of each candidate colliding wind system and look for recurring orbital-phase-related variations. Such a study is underway.