An oleo strut is an 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. 
The largest cargo airplanes in the world, like the Antonov An-124 Ruslan, use oleo struts to allow for rough-field landing capacity with a payload of up to 150 tons, and have a kneeling mechanism which allows the fuselage to lower for unloading. 
An oleo strut consists of an inner metal tube or piston, which is attached to the wheel axle, and which moves up and down in an outer (or upper) metal tube, or cylinder, that is attached to the airframe. The cavity within the strut and piston is filled with gas (usually nitrogen, sometimes air – especially on light aircraft) and oil (usually hydraulic fluid), and is divided into two chambers that communicate through a small orifice.
When the aircraft is stationary on the ground, its weight is supported by the compressed gas in the cylinder. During landing, or when the aircraft taxis over bumps, the piston slides up and down. It compresses the gas, which acts as a spring, and forces oil through the orifice, which acts as a damper. A tapered rod is used on some designs to change the size of the orifice as the piston moves, providing greater resistance as compression of the strut increases. Additionally, a check valve is sometimes used to uncover additional orifices so that damping during compression is less than during rebound.
Nitrogen is usually used as the gas instead of air, since it is less likely to cause corrosion. The various parts of the strut are sealed with O-rings or similar elastomeric seals, and a scraper ring is used to keep dust and grit adhering to the piston from entering the strut.
Hydropneumatic suspension, introduced in 1954, utilizes the same principle of a gas that compresses (nitrogen) and a fluid that does not. In this application, an engine driven pump is used to pressurize the hydraulic fluid.
- FedEx Express Flight 80 - bounce leading to crash
- Van Sickle, Neil D., Welch, John F, Bjork, Lewis and Bjork, Linda, "Van Sickle's modern airmanship." Page 125. Retrieved March 12, 2011
- Woodhouse, Mary and Gifford, Scott "How to make your airplane last forever," Tab Books, 1996, Page 75. ISBN 978-0-07-071704-6. Retrieved March 12, 2011