The operation phase is not a phase in the project life cycle, because it starts after the project is completed. The operation phase is the phase when the product/service (resulting from the project) starts to be used (in other words, in operation mode).
The operation phase is also referred to as the maintenance phase.
For example, let's assume that your project was about creating an accounting software. Once the project is done, and the accounting software starts being used by the intended users (the accountants), then you are in the operation phase.