You're on the right track! Here's the breakdown of the levels of organization in living things, including the ones you mentioned:

1. Cells: The basic, fundamental unit of life. All living organisms are made up of cells.

2. Tissues: Groups of similar cells that work together to perform a specific function. Examples include muscle tissue, nervous tissue, and connective tissue.

3. Organs: Structures made up of different tissues that work together to perform a complex function. Examples include the heart, lungs, and stomach.

4. Organ Systems: Groups of organs that work together to carry out major bodily functions. Examples include the digestive system, circulatory system, and nervous system.

5. Organism: A complete living being made up of multiple organ systems. This is the highest level of organization.

So, the missing piece in your list is Organ Systems.