O5, at least in the Army, is a often the rank of battalion commander. The battalion is a significant maneuver element within the combat organizational structure and the first element to have combined arms integration for command and control purposes. To use the common vernacular, where the rubber meets the road. It is also the worker level for Division staff. The General Staff elements, G-1 through G-5, are usually O6 billets for the head of the staff section, but that usually ends up managing and coordinating with those in the grade below doing the execution of work. Back in the 80s and early 80s, the Army officer evaluation reporting system had the senior rater as the Division commander for the battalion commanders and senior members of his general staff. Those annual [and change of command] reports could make or break a career, if your career goals were further advancement up the chain of command. It would influence personnel management actions such as selections for War College and joint assignments, multi-service or multi-national, where because of the Goldwaters-Nichols Act were required for senior command positions.

And we all know how that exercise turned out.

Regards

JJ