Water is a molecule of 2 parts (2 atoms) hydrogen, 1 part (1 atom) oxygen, unless it undergoes electrolysis into elemental hydrogen and oxygen, it will remain H2O. Water can have dissolved oxygen (why fish can respire in water), but his does not affect the potablity of the water. Yes, it does affect the “taste”, which is why it is often recommended to pour boiled water from container to container, which will introduce dissolved oxygen and improve the taste. If the water is sterile and is maintained in a sterile sealed container, it cannot go “bad”. This additive, likely is, like adding hydrogen peroxide, in which a small amount of nascent or free oxygen is generated which has bacterialcidal properties.

Pete