Due to lack of ranking structure document (at least as far as I could find) I have compiled a complete ranking structure.
NOTE: This does not include special forces or any custom jobs within Omi9.
Recruit, Private, Private First Class, Specialist, Corporal, Sergeant, Staff Sergeant, Master Sergeant, Sergeant Major, Warrant Officer, 2nd Lieutenant, 1st Lieutenant, Captain, Major, Lieutenant Colonel, Colonel, Vice Commander, Commander.
Shortened: RCT, PVT, PFC, SPC, CPL, SGT, SSGT, MSGT, SM, WO, 2LT, 1LT, CPT, MAJ, LTCOL, COL, VCMDR, CMDR.
Shoutout to @Guinea Pig (again lol) for helping compile the list.)
Edit: Mistakes pointed out by @Right Twix Bar @Rangiatea