Let's take a moment to practice assigning oxidation numbers in the context of the oxidation of aldehydes. Potassium permanganate (KMnO4) or chromic acid (H2CrO4) oxidize aldehydes (and alcohols) to form carboxylic acids. In the initial state in potassium permanganate, the oxidation state of manganese is +7 and in chromic acid, the oxidation state of chromium is +6. In an aldehyde, the oxidation state of carbon is +1. After the reaction, the carboxylic acid carbon will have an oxidation state of +3.

Do you understand the rationale behind the assignment of a +1 oxidation state to aldehyde carbon and +3 to carboxyl carbon? Look at the covalent bonds and assign control of the electrons based on electronegativity, and you will understand.

Through the oxidation of an aldehyde, manganese or chromium will gain electron control while carbon will lose electron conttrol. For example, with KMnO4, two equivalents for every aldehyde will be reduced in oxidation state from +7 to +6 (MnO4- becoming MnO4-2). Carbon changes from +1 to +3 in being oxidized.