Originally Posted by flightlogic
Looks pretty old. You will need to have it hydro tested... for safety.
Correct. However, it might be current as-is. Can we see a photo of the tank neck markings? If it's ever had a hydro test since manufacture, then stamped into the neck there should be a one or two number month, with some (probably 4) smaller numbers stacked in the middle, followed by a two-number year. To be refilled or transported with compressed gas on US public roads, the last hydrostatic test date must be no more than 5 years ago (this timeframe can be different for composite cylinders, but DOT 3AA and 3AL cylinders are almost universally 5 years). You could fill it yourself from a welding cylinder without a current hydro, but you're sort of taking your chances with a bomb.
Because of the rounded bottom, you know it's a steel cylinder. Another way to know it's steel is its DOT designation. DOT 3AA cylinders are steel.
Getting a tank tested isn't difficult, but it can be costly relative to the price of a new modern cylinder at about $25 around here. That $25 would go a decent ways toward a new lighter-weight aluminum tank with current hydro.
Also, that valve looks like a standard keyless CGA870 medical O2 valve.