Oxidation can be defined as a reaction in which atoms react with oxygen [well THAT figures!] but this definition is far too unrefined! Many reactions that do not involve oxygen at all may still be classified as involving oxidation. To broaden the application the reaction may then be defined as one involving loss of electrons. However, it isn't long before even this fails as atoms can "lose" different numbers of electrons.
The best definition therefore involves the concept of "oxidation number"..which can be defined as: "The formal charge that an atom would have if the bonding in the chemical species were considered to be wholly ionic". By the way, this is a definition that the examiner expects you to know. So learn it. In an oxidation reaction the oxidation number of an atom will increase...become more positive...as the atom loses units of negative charge [electrons].
Oxidation CANNOT happen in isolation...and its partner REDUCTION must occur simultaneously. Reduction will therefore absorb the electrons that the oxidation liberates. As a result the oxidation number will become smaller [more negative or less positive] as the atoms acquires units of negative charge [electrons]
In a REDOX recation then all of the electrons produced in the oxidation MUST be consumed in the accompanying reduction. There can be NO spare electrons!