How to obtain sodium oxide from sodium chloride?
There is no need (or possibility, really, in terms of standard lab capabilities) to oxidize sodium(I).
In fact, one method relies on sodium(I) reduction to metal as a method of eliminating unwanted chloride.

Method 1


Electrolysis of molten sodium chloride:
$$\ce{2 NaCl(l) -&gt; 2 Na(l) + Cl2(g)}$$
Oxidation of sodium metal to oxide by burning:
$$\ce{4 Na + O2 -&gt;[&gt;\pu{250 °C}] 2 Na2O}$$
Drawback: pure sodium oxide cannot be obtained by direct oxidation of sodium.
Instead, a mixture of sodium peroxide and sodium oxide is formed.
In order to suppress the formation of peroxide, sodium metal or sodium nitrate is added in excess to the mixture in inert atmosphere:
$$\ce{Na2O2(s) + 2 Na(l) -&gt;[\pu{150 °C}] 2 Na2O(s)}$$


Method 2


Convert sodium chloride to sodium bicarbonate using Solvay process:
$$\ce{NaCl(aq, conc) + H2O(l) + NH3(g) + CO2(g) -&gt; NaHCO3(s) + NH4Cl(aq)}$$
Thermal decomposition of bicarbonate first yields in sodium carbonate:
$$\ce{2 NaHCO3(s) -&gt;[\pu{250 - 300 °C}] Na2CO3(s) + CO2(g) + H2O(g)}$$
which is subsequently calcined to form an oxide:
$$\ce{Na2CO3(l) -&gt;[&gt;\pu{1000 °C}] Na2O(s) + CO2(g)}$$
This appears to be a preferred method as it is less energy- and resources-consuming and allows to obtain $\ce{Na2O}$ selectively.