Reduced ... oxidized.....
Oxidation is the increase in oxidation state. Reduction is the decrease in oxidation state.
.-3+1......0..........0.........+1-2 ........... oxidation states
4NH3 +3O2--> 2N2 + 6H2O
As you can see, nitrogen is oxidized, and oxygen is reduced.
The oxidizing agent "causes" the oxididation and is the reactant which includes the element reduced. The oxidizing agent is O2 gas. The reducing agent "causes" the reduction and is the reactant which includes the element oxidized. The reducing agent is ammonia, NH3.
Also, keep in mind that oxidation states are not actual charges. For instance, in ammonia, nitrogen atoms do not carry a charge of -3. Nitrogen is estimated to have a charge of about -1.1, while each hydrogen has a charge of about +0.36. There is no complete transfer of 3 electrons to nitrogen. The transfer of electrons, as used in the description of oxidation and reduction, and in half-reactions, is hypothetical. It is a convenient way to balanced redox reactions, but it does not reflect the reality of oxidation and reduction.
2(2NH3 --> N2 + 6H+ + 6e-) ............ oxidation
3(O2(g) + 4H+ + 4e- --> 2H2O) ....... reduction
---------------- ---------------- -------------
4NH3 + 3O2 + 12H+ --> 2N2 + 12H+ + 6H2O
4NH3(g) + 3O2(g) --> 2N2(g) + 6H2O(g)
The hydrogen ions are strictly hypothetical since the reaction takes place in the gaseous state and are used only as an aid in balancing the half-reactions.