the invention relates to an oscillating piston engine comprising a housing that contains a first and at least a second piston , both of which can rotate about a fixed rotational axis in the housing and which when rotating about said axis execute counter - rotating displacements back and forth about a pivoting axis that runs perpendicular to the rotational axis through the centre of the housing . a working chamber is situated between opposing end faces of the first and at least second piston . the housing is provided with at least one gas exchange opening for admitting or discharging a gas into or from the working chamber . the pistons are arranged in such a way that the rotational axis runs through the working chamber . the pistons are mounted to slide in a piston cage , which is located in the housing , is concentric with the rotational axis , and rotates together with the pistons about said axis . the gas exchange opening is arranged eccentrically in relation to the rotational axis , on one end face of the housing .