By Gum:

[KO-Rec] Recruit
[KO-Inf] Infantry
[KO-Sgt] Sergeant
[KO-Lt] Lieutenant
[KO-Capt] Captain
[KO-Major] Major
[KO-Col] Colonel
[KO-Gen] General
[KO-Vet] Veteran

By McBurger:

[KO-LG] KO Legend highest rank
[KO-EO] KO Extreme ownage second highest
[KO-MF] KO Master Fighter
[KO-SA] KO Strong Attacker
[KO-AA] KO Average Attacker
[KO-NA] KO New Attacker (for new recruits)

By Setcomx:

KO-CP]-couch patator XD
[KO-B]-Basic
[KO-A]-advanced
[KO-I]-invader
[KO-SL]-semi leader
[KO-L]-Leader
[KO-KOF]-kaofreak