Inertial and wall effects on the motion of a ball oscillating in a rigid container are investigated both experimentally and theoretically. Experiments are performed with an oscillating ball rheometer to determine the effects of inertia and of the wall on the motion of a rigid sphere oscillating within a Newtonian fluid in a vertically oriented cylindrical tube. It is found that under certain conditions the effect of inertia on the drag force acting on a ball oscillating inside a cylindrical tube is well described by the theory for a ball oscillating inside a spherical container. When the imaginary component of the total hydrodynamic force is negligible, this result allows a correction and the use of the Stokes drag equation to account for the boundary and frequency effects in the determination of the non‐Newtonian viscosity with a ball rheometer. Finally, it is shown that a simple study on Newtonian fluids can provide information to help understand the more complex effect of inertia in non‐Newtonian suspensions. As an application, rheological properties of concentrated red blood cell suspensions are studied in a ball rheometer at frequencies of oscillation where inertia is important. It is found that additional dependence of the storage modulus on inertia is not proportional to the square of the angular frequency of oscillation as stated by Sellersetal., and that terms inSof order higher than two are not negligible in the case of a ball rheometer.