Ranks
There are only two notable ranks in Onmitsukido. On the top is the Commander in Chief. The Commander in Chief is in control of Onmitsukido and is also the Captain of Second Division and they also have their own division. Then there is called the Corps Commander. The Corps Commander corresponds with seated officers 1-5.
Onmitsukido Divisions
1: The Executive Militia, and is also known as the "Punishment Squad." Not much is known about this squad, though it's alternative name may give a clue.
2: Patrol Corps. This division is responsible for patrolling the various lands and dimensions to combat dangerous Hollows and other enemies of Soul Society
3: Assassination Squad.
4: Detention Unit. This unit is responsible for the imprisonment and supervision of all criminals within the Seireitei. They also have another duty known as Special Detention which was related to those who joined the Gotei 13 but who were judged to be a danger to their fellow Shinigami. They would investigate, apprehend, and keep them sealed away from the rest of Soul Society within the special underground detention facility aka. the Nest of Maggots.
5: Reversal Counter Force: This division counters any invasions threatening the Soul Society, using any means necessary.
Onmitsuko Uniforms
Although the general Onmitsukidō uniform appears to be the standard Shinigami uniform, the Executive Militia uniform is different from the Gotei 13. Overall, it resembles a ninja uniform. It has no visible white undergarments and the sash is dark as opposed to white. A headpiece covers the bottom part of the face and sometimes the top, but not the eyes; they wear no sandals, but long jika-tabi , and tight bands are on the legs and arms to keep the clothes from moving too much, which both are presumably for silence of movement. In the anime, members of the Executive Militia also wear their head covering on the top, not just the bottom and wear a black sash instead of a white one.
The leader of the militia wears the same uniform, except that the shoulders and back are exposed. The reason for the empty space is the eventual usage of Shunkō , an advanced technique that combines hand to hand combat and kidō, causing high spiritual pressure to accumulate at the shoulders and back, blasting away the fabric at said places.
The Correctional Force uniform is radically different. The soldiers wear a white uniform, a long piece of headgear, and a backpack.
Information gathered from Bleach Wiki
