[sebbysteiny]

Hmmmm... Oxygen absorbers... tell me what all you know...

Erm, only what I sent you in the links. They are used as food preservitives, usually work using iron oxide and are quite cheap. The rest is for you.

[cockydude]

There is this weird method that actually does vaguely measure Oxygen content but it will pretty accurately rank up oxygen production.
Get a water plant (hydrilla?) then put it inside an inverted boiling tube with water to the brim. Then after sometime measure the amount of air bubbles around the plant. Tedious, but not impossible.