While environmental epidemiology is one aspect of environmental health, OEH is quite a broad field that encompasses many others. The range from toxicology, risk assessment, emergency management, environmental hygiene, to environmental epidemiology, there are many directions to go in. Each school likely offers its own strengths within OEH.
Epidemiology is the research tool of public health. It has many sub-concentrations within epidemiology such as cancer, cardiovascular, infectious, and other therapeutic areas. But the tools mostly remain the same, and the methodology and training is similar with differences between each sub-discipline.
In general, I think there's probably more jobs available in epidemiology because it's a more commonly used tool. However, many many more people train in it, so the competition is going to be much greater. OEH has relatively few trainees, so there are relatively smaller number of jobs, as well.
Hope that helps.