Do I have to include on-site incidents in my accident history?

If I have a large on-site incident, but no offsite impact, would I have to report it in the five-year accident history?

It would depend on whether you have onsite deaths, injuries, or significant property damage. You could have a large accident without any of these consequences (e.g., a large spill that was contained); this type of release would not have to be included in the five-year accident history.