Overloading operators, that is, giving operators such as + or - new or additional functionality, is not inherently object-oriented, but since it can be thought of as adding polymorphism to functions it fits in naturally here after we have gone through polymorphism through generic functions.
Opinions vary on whether overloading operators is good or bad practice. Some languages allow it; others do not. Some languages allow you to make your own infix operators but not change existing ones, and some languages are just inconsistent in allowing some operator overloading for built-in objects ...
