-Noach agreed to save Og on the condition he would remain his servant.
-He allowed him to ride on top of the Ark and created an opening through which he could enter his head when the atmospheric conditions became too rough.
-Noach saved him, since the age of Mosshiach had not arrived yet and the klipas still had to exist.
-In addition there is the concept that the klipas should not be destroyed but rather used for good. This is why he bound him as a servant and he was later given over to Shem.
-Og represented the physical manifestation of the klipas.
-The world was irrelevant during the flood and the Ark represented the concept of a ‘miniature world’.
-In addition there were other aspects of the klipah that were represented by Chum his wife and the raven.
-Since the klipah are always the surrounding shell, Og had to stay on the outside of the Ark.

(Ramchal on Torah, Noach)