Gain and Loss of Oxygen
The earliest view of oxidation and reduction is that of adding oxygen to form an oxide (oxidation) or removing oxygen (reduction). They always occur together. For example, in the burning of hydrogen . The ionization energy measures how hard it is to lose or remove an electron. High ionization energy means that it is hard to lose electrons. Low ionization energy means that it easy to lose electrons.
The elements on the left side lose their electrons fairly easily and the elements on the right side of the periodic table do not lose their electrons very easily. Taking vertical position on the table into account, the elements that are lower on the table lose electrons more easily and the elements that are higher have a harder time losing electrons. Thus the overall trend is from most easily losing electrons on the lower left to least easily losing electrons on the upper right. Keep that trend in mind.
2H2 + O2 -> 2H2O
The hydrogen is oxidized and the oxygen is reduced. The combination of nitrogen and oxygen which occurs at high temperatures follows the same pattern.
N2 + O2 -> 2NO
This formation of nitric oxide oxidizes the nitrogen and reduces the oxygen. In some reactions, the oxidation is most prominent. For example in the burning of methane,
CH4 + 2O2 -> CO2 + 2H2O
both carbon and hydrogen are oxidized (gain oxygen). The accompanying reduction of oxygen is perhaps easier to see when you describe reduction as the gaining of hydrogen.
On the other hand, the reaction of lead dioxide at high temperatures appears to be just reduction.
2PbO2 -> 2PbO + O2
The reduction of the lead dioxide is clear, but the associated oxidation of oxygen is easier to see when you describe oxidation as the losing of electrons.