Refer to figure.
The figure shows an oleo strut which is a type of oleo-pneumatic piston used in landing gear. It is filled both with oil and some air/charged nitrogen.

• The gas provides immediate shock absorption, especially when loaded during landing. The gas is easily compressible, so acts as a spring in the mechanism (it provides elasticity).
• The oil is used to for damping - slowing the rate at which the piston extends and retracts. During rebound (which is when the load is removed from the piston) the oil slows the rate at which the piston extends again.

Without the oil, the piston may oscillate too rapidly. The oil also increases the resistance due to the gas as the piston becomes compressed, by flowing through an orifice and reducing the volume in which the gas is. Without the gas, the incompressible oil would not absorb much energy from impacts, and instead transmit them to the rest of the structure, causing damage and possibly injury to the occupants.