In protozoa like Amoeba and Paramecium, an organ is found for osmoregulation which is : -

Freshwater protozoans such as Amoeba and Paramecium live in a hypotonic medium, so water keeps entering their cells by osmosis.
The contractile vacuole collects the excess water from the cytoplasm and discharges it to the outside at intervals, so the cell does not swell and burst. This is osmoregulation.
Mitochondria produce energy, the nucleus controls cell activities, and food vacuoles digest food.
So the organelle for osmoregulation is the contractile vacuole.