Organ is a collection of tissues combined in a structural unit to perform a common function.
Organ system is a unit in which many organs work together to carry out one particular function. For example- cardiac cycle is carried out by heart, blood vessels and blood.
Cell aggregate is an aggregation of cells at a particular site in a particular situation, e.g. when blood vessels being ruptured then platelets become aggregated at that site.
A group of cells having a common origin and performing a similar function is called as tissue.