According to the explanation given in the previous session, for this question we have:
4 Fe + 3 O2 -> 2 Fe2O3
The tip to find oxidation number is always rely on "easy" elements, like halogens or Oxygen, which will most of the times have -2 as its oxidation number. With that information in mind we have 50% of the answer already, since Oxygen will form compounds carrying a -2 charge
Oxygen will be reduced, it will also be the oxidizing agent, oxidation number = -2 and the change in oxidation number will be 0 -> -2.
The rest of the answer is easy now
Iron will be oxidized, becoming the reducing agent, oxidation number must match the oxidation number for Oyxgen, therefore it will be = +3, change in oxidation number 0 -> +3