IGCSE Science/Section 2: Structures and functions in living organisms/Levels of organisation
Individual cells are made out of even smaller parts called organelles. Multicellular organisms have tissues: groups of similar cells. Tissues make up organs, which are groups of different types of tissues adapted for a particular function. Organs working together can perform a specific process, they are called organ systems.
An example in the heart: Nucleus (organelle) > Muscle cell > Cardiac (heart) muscle tissue > Heart > Circulatory system > Human
So generally the order is: Organelle > Cell > Tissue > Organ > Organ system > Whole organism