[dmurray]

I think you're double counting; you need 1.25 kg of oxygen and nitrogen combined to replace 1 cubic metre of air.

1.2t of candles doesn't seem like an unreasonable amount of extra payload if they would really be valuable in an emergency. The ISS weighs 400 tons and a napkin estimate says it has had 1000 tons of resupply missions. The candles have a shelf life of 10+ years.

[sam1714]

You need 1.25 kg of gas. Candles don't consist solely of oxygen, the ones the Russians use utilize lithium perchlorate (LiClO4). When they finish burning, you are left with a lithium chloride ash (LiCl), which will be 40% mass of what you started with.

It works out to be more efficient, at least in terms of mass, to send up large tanks of compressed gas instead.