We present an experimental setup (OLGA: Oxygen Low Gravity Apparatus) designed to compensate, partially or totally, gravity forces in a sample of O-2 by means of magnetic forces. The compensation remains within 1 % in a 5 cm(3) volume and within 5 % in a 40 cm(3) volume. The experiments can be carried out at intermediate controlled gravity levels, between Earth (1g(0)) and zero (0g(0)) gravity by adjusting the magnetic field intensity. The setup can also be used to perform fast variations of gravity, with a time constant of 340 ms, from -0.5g(0) maximum (overcompensation) to 0g(0), or from 0g(0) to 0.4g(0) maximum. The sharp acceleration or deceleration of spacecrafts can thus be reproduced to investigate in detail for instance, the transient behavior of life support systems or, when properly rescaled, fuel in rocket tanks. Numerical simulations and experiments also show that high conductive materials (copper) near the cell can modify the magnetic forces during the transient and then the accelerations. In contrast, low conductive materials (titanium) have negligible effect.