Answer:
OS - Ordinary Seaman
AB - Able Seaman
LS - Leading Seaman
MS - Master Seaman
PO2 - Petty Officer 2nd Class
PO1 - Petty Officer 1st Class
CPO2 - Chief Petty Officer 2nd Class
CPO1 - Chief Petty Officer 1st Class
NCDT - Naval Cadet
ASLT - Acting Sub-Lieutenant
SLT - Sub-Lieutenant
Lt - Lieutenant
LCdr - Lieutenant-Commander
Cdr - Commander
Capt - Captain
Cmdr - Commodore
RAdm - Rear-Admiral
VAdm - Vice-Admiral
Adm - Admiral