There isn't a single, commonly used word for a cell that can live on its own that starts with "o". However, here's why and some options:

* "Organism" is the most accurate term for a living entity that can exist independently. While it refers to a whole being, it's built of cells.

* "Organelle" starts with an "o" but refers to a functional unit within a cell, not a cell itself.

* "Unicellular" describes an organism composed of a single cell, which fits the description you're looking for.

It's important to understand that while many single-celled organisms can survive independently, there are some that rely on other cells for survival.

Let me know if you have any other questions about cells or organisms!