Oleo strut

An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations.

It is undesirable for an airplane to bounce on landing - it could lead to a loss of control. The landing gear should not add to this tendency. A steel coil spring will store impact energy and then release it - the impact energy being the force of the airplane hitting the ground. An oleo strut absorbs this energy, reducing bounce.