How much does oxygen weigh?

Just like any other gas, that depends on what size jar of it you have (volume),

its temperature, and how tight it's packed into the jar (pressure).

"Standard" temperature and pressure means the temperature of freezing water

and the pressure of air at sea level.

In those conditions, the density of oxygen is 1.429 grams per liter.

So 1 cubic foot of it would weigh 1.427 ounces. (rounded)