Oxygen tanks are found among the general public related to the medical use of oxygen and recreational scuba diving. And therefore inevitably a certain number of oxygen tanks end up misplaced, lost or discarded every year. Oxygen tanks are not trash, however, and do not belong in the waste stream. They are usually made of high-quality steel and can be reused hundreds of times, and even when eventually worn out should be recycled and not thrown away.
Examine the oxygen tank for the owner's name. If the tank is owned by a business like a dive shop or a hospital, the name is usually displayed prominently, but if it is owned by an individual the name may be in small letters on the bottom or near the neck or valve. If you can identify the owner, it is preferable to return the tank to them (even if it looks old or worn out) as it is their property.
Take the oxygen tanks to your local community or county recycling centre to be recycled if they accept oxygen tanks. In some cases they have arrangements with local businesses to come pick up all old oxygen (and other gas) tanks, and in other cases they clean and add the tanks with their other scrap metal for recovery by melting, but sometimes they simply do not accept them. At least they can usually direct you to local businesses who will accept the tanks.
Create art or landscaping projects with the old oxygen tanks. Some creative ideas include bells made from oxygen tanks, and found object art incorporating old oxygen tanks.