Oxygen gain must be oxidized and oxygen loss must be reduced, right

Oxygen gain must be oxidized and oxygen loss must be reduced, right

By all means.
The original concept of redox reaction is that substances get oxygen and lose oxygen
It is found that when a substance gets oxygen, it must lose electrons; When a substance loses oxygen, it must get electrons. Therefore, redox reaction is extended to any reaction with electron gain and loss, which is generally called redox reaction

The following statement is correct: a hydrogen peroxide solution contains oxygen; B hydrogen peroxide reacts with manganese dioxide to release oxygen

Both are wrong
Hydrogen peroxide solution decomposes under the catalysis of manganese dioxide to release oxygen

In the following description of the combination reaction, the correct one is () A. The combination reaction must be an oxidation reaction B. The reaction with oxygen must be a chemical reaction C. A reaction in which oxygen must participate and only one substance can be produced is a combination reaction D. A reaction in which two or more substances produce another substance is called a combination reaction

A. The combination reaction is not necessarily an oxidation reaction. For example, carbon dioxide reacts with water to form carbonic acid. This reaction conforms to the characteristics of "polytropic one". It belongs to the combination reaction, but it is not a chemical reaction between material and oxygen, not an oxidation reaction, so the option statement is wrong
B. The reaction with oxygen is not necessarily a combination reaction, such as methane + oxygen ignition
Carbon dioxide + water, the products of this reaction are two kinds, which do not accord with the characteristics of "polytropic one", and do not belong to chemical reaction, so the option statement is wrong
C. Oxygen does not necessarily participate in the combination reaction. For example, carbon dioxide reacts with water to form carbonic acid. Without oxygen, the reaction belongs to the combination reaction, so the option statement is wrong
D. The reaction of two or more substances to produce another substance is called chemical reaction
Therefore: D

The following statement is correct () A. The oxidation reaction must be a chemical reaction B. The reaction of two or more substances is a chemical reaction C. Only when a substance combines with oxygen is oxidation D. The combustion of magnesium strip in air belongs to both chemical reaction and oxidation reaction

A. Oxidation reaction is a reaction of increasing valence. Combination reaction is a reaction in which two or more substances form a substance. The two are not directly related. Some combination reactions belong to oxidation reaction, but the oxidation reaction is not necessarily a combination reaction, so a is wrong;
B. When two or more substances react to form a substance, it belongs to chemical reaction, so B is wrong;
C. The essence of redox reaction is electron transfer, which is characterized by the change of valence, which has nothing to do with the presence of oxygen, so C error;
D. Magnesium reacts with oxygen to form magnesium oxide, which belongs to chemical reaction. The change of valence belongs to redox reaction, so D is correct
Therefore, D

Is it right to say that a reaction in which two substances produce one substance is called a combination reaction?

No, because as long as two or more substances form a substance, the reaction is called combination reaction

Must the chemical reaction with oxygen be oxidation

The reaction with gain and loss of electrons is redox reaction. If something oxidizes, something reduces, because electrons are conserved. So, now, with oxygen, it is redox reaction