The Order of the Arrow (OA) is a nationwide organization. Of course, covering 50 states would be difficult without having smaller organizations, kind of like the US has states, counties, and cities.
In Scouting, we have troops (or units) which are part of districts which are part of Councils. There are other levels, but they don't matter as much.
In the OA, we start with the Chapter. Our chapter is Migisi Opawgan chapter. There are 29 chapters in our lodge. Each chapter serves the units in one or more districts.
- Right above the chapter is the Service Area; this is a group of several chapters. There are 5 service areas in our lodge and they are based on geography. Migisi Opawgan chapter is in the Southern Service Area.
- The 5 Service Areas make a Lodge. Our lodge is Mishigami Lodge and our lodge serves the Michigan Crossroads Council.
- Right about the lodge is the Section and beyond the section the only group that usually matters is the National Order of the Arrow.
It's probably worth noting that not all lodges have chapters or service areas, but since Mishigami Lodge serves nearly the whole lower peninsula of Michigan, it makes sense to have smaller groups do the work.