Reaction with metals:
Most metals burn in oxygen forming basic oxides.
〖2Mg〗_((s))+O_(2 (g))→〖2MgO〗_((g))
    Reaction with non-metals:
Most non-metals also burn in oxygen forming oxides most of which are acidic oxides.
C_((s))+O_(2 (g))→〖CO〗_((g))
S_((s))+O_(2 (g))→〖SO〗_(2 (g))
    Rusting:
Rust is hydrated iron III oxide. (Fe2O3.XH2O)

Rusting will only take place in test tube because air and water are necessary for rusting. It can be prevent by painting, vanishing, greasing and galvanization or coating.
    Combustion:
Oxygen supports combustion. I.e.: Substances burn in oxygen (e.g. Methane) to give water and carbon dioxide.
〖CH〗_(4 (g))+〖2O〗_(2 (g)) 〖2H〗_2 O_((g))+〖CO〗_(2 (g))
    Photosynthesis:
Oxygen is a product of this process.
〖6CO〗_(2 (g))+〖2H〗_2 O_((l))→C_6 H_2 O_(6 (aq))+〖6O〗_(2 (g))
    Respiration:
This is the reverse of photosynthesis. O2 combines with glucose to produce carbon dioxide, water and oxygen.
C_6 H_12 O_(6 (g))→〖6CO〗_(2 (g))+〖6H〗_2 O_((l))+energy
    Oxygen is involves in many important processes

Uses of Oxygen
    Oxygen is use in the manufacture of steel.
    It is use in oxyacetylene welding.
    Oxygen is use in breathing apparatus in hospitals and by deep-sea drivers.
    It is use as a fuel oxidant in rockets.