An organ is more complex than a tissue because although tissues and organs are both made up of cells, organs are made up of different tissue groups. There are specific tissue groups for the structuring of different organs, so it would be contradictory to say that a tissue is more complex than an organ since the organ is made up of more cells and a variety tissues whereas the tissue is made out of cells only. An analogy could be that a series of gears make up an engine so the engine would be more complex rather than vice versa.