Ozone, O3 , is a gas that is effectively oxygen, O2 , with an extra atom. In this state it is a very unstable gas, but a very strong oxidizer. Ozone is manufactured from oxygen by high voltage spark. The smell in the air after a close lightning strike is ozone. As an oxidizer, ozone will break down cell walls of bacteria and viruses and as such kill and destroy them. Ozone is however a very unstable and will naturally start reverting back to oxygen from which it was formed.
So why add it to water? A proper water purification system will eliminate all bacteria and viruses, but during filling of bottles, bacteria from surrounding environments (empty bottle, cap, equipment, air) may be introduced in the water and it will then grow in the water. To prevent this, ozone is introduced in a controlled environment in a narrow band of 0.2-0.4 ppm (parts per million) into the water. In this dissolved form in the water, it will kill any bacteria that may have been introduced. Once sealed, water should be good for more than a year.
After sealing the bottle, the water is safe, and the ozone starts reverting to oxygen. If dissolved in water, it has a “half-life” of 30 minutes, meaning every 30 minutes the quantity is halved, in other words, back to oxygen. As such bottled water is always kept for at least one day on the warehouse floor to ensure that all the ozone has reverted back to oxygen. Ozone in water will have a pungent smell and water should not be consumed while still present.