Most high mass X-ray binaries contain neutron stars as companions to an OB star, while high mass black hole binaries are very rare. We use rapid binary population synthesis to predict the number and properties of OB stars with compact companions, while varying uncertain physics assumptions. We find that synthetic populations which agree with the population properties of Be stars, Wolf-Rayet stars, and neutron stars forecast a large and so far undetected population of massive black hole binaries with orbital periods between a few days and 1000 days. To find or rule out this population is key for quantifying the contribution of isolated binaries to the merging massive black holes found through their gravitational wave emission.