This is an original Oster replacement part.
The purpose of this item is to hold the blade and seal in place at the bottom of the blender jar
Because it has plastic threads they will wear at times and the jar will leak.
Over tightening may crack this part, dropping it may break it and excessive heat may warp it.
Once you have this off, this is a good time to check the sealing ring to see if it is not cracked or if it has been compressed too much.