An organ is different tissues (which is made from cells) working together to perform different functions. Example of organs include liver, lungs, pancreas, leaf, fruit etc.
Organ system is a group of different organs working together, again performing different actions. Examples of organ system includes the digestive system (which includes organs like stomach, pancreas, intestine...) or the excretory system (which includes organs like kidney, bladder...)
Remember that organ system is made from organs, organs are made from tissues, tissues are made from cells, and cells are made from organelles. They form a sequence.