Well, I think the term is used a bit loosely in gyms but an actual OTed state was mentioned above and is one of a systemic nature, not just a group and or muscle training per se.
I think Siff went into this in Supertraining.
An actual OT state is not easy to bounce back from, or at least not a few deloads and you're fine. Chronically OTed effects the system in a breaking down way.
Effect and or impairment "can be" in...
Coordination possibly
Blood pressure
Endurance
Sleep requirements
RHR
Body temp
Appetite
Metabolism
Muscle aches
Resistance with immune system
Recovery time
Emotional changes