OTS and ME/CFS are both conditions that seem to result from chronic physical and emotional strain, and lack of appropriate resources for recovery and restoration. OTS behaves like a miniature form of ME/CFS, in that the damage is generally caused directly by over-training and under-recovery which lead to mitochondrial damage (oxidative stress) and nervous system dysregulation.
The primary difference between the two is that recovery is expected from OTS within 3-12 months. With OTS, the body is still able to respond to rest, repair the damage, and heal. With ME/CFS, the story becomes more complicated. The body may not heal, or it may require much more help than even the most dedicated rest.
In my case, over-training was the stress that kicked off a chain reaction. A history of disordered eating collided with a two-year period of over-training, under-recovery, and acute emotional distress. While a healthy body normally harbors pathogens and toxins kept suppressed by the immune system, the stress of OTS rendered my body unable to keep such pathogens at bay, and old infections such as Mononucleosis and Lyme Disease were able to resurface.