OTS is almost certainly the same as chronic fatigue syndrome. The only difference is that in OTS the stressors are usually mostly physical, where in CFS they are (usually) mostly mental. In many cases there is also an apparent flu-like illness at onset, although it isn't clear whether this is an actual illness or just a symptom of the immune system going haywire.<p>It seems that at some point after prolonged, chronic stress the brain just says "that's enough", and further stressors result in a reduced stress response from the brain, rather than the usual increase in stress response (cortisol, etc.) Essentially it's a form of burnout.