Here's a quick cut n' paste from Wikipedia:
Na2O forms when sodium is treated with oxygen.
4 Na + O2 → 2 Na2O
Burning sodium in air will produce Na2O and about 20% sodium peroxide Na2O2.
6 Na + 2 O2 → 2 Na2O + Na2O2
Pure Na2O can be prepared by reaction of liquid sodium with NaNO3.
10 Na + 2 NaNO3 → 6 Na2O + N2
In the case of burning sodium in air, what you'll get isn't rigidly defined by reaction conditions, as I recall, perhaps it's an equilibrium between Na2O and Na2O2 ?