The main organ in the excretory system is the kidney. The kidney is the main organ that controls the excretory systems for vertebrates. It is also part of the Urinary system which is part of the excretory system.
A respiratory organ would be an organ that aids in the process of respiration/breathing i.e. the lungs. An excretory organ is an organ that aids in the process of excreting urine or feces i.e. the bladder.