A comprehensive investigation on the impact of piston design on scavenging and combustion in an opposed- piston, opposed-cylinder (OPOC) two-stroke engine is carried out and presented in this paper. Two-stroke engines, in general, have superior power densities and brake thermal efficiencies. Compared with opposed-piston (OP) engines, the OPOC architecture comprises only one crankshaft instead of two, and all the forces generated on the piston go to this one crankshaft via a common bearing, thus making the engine structure inherently simple, lightweight, compact and efficient. Due to the piston motion of the OPOC engine, two opposing injectors were mounted at the center of the cylinder wall for each cylinder. This unique feature posed challenges on air entrainment for air/fuel mixing because of the inherent limited space for injection spreading angle near top-dead-center (TDC). Motivated in this regard, a novel torus chamber design (a combination of both pistons which form a ring-shaped combustion chamber at TDC) was adopted, and has been proved to be more efficient in air utilization for combustion. In the design process of the piston for the OPOC engine, other factors such as gas scavenging, piston thermal management, which are critical to engine performance and durability, are also taken into account.In this work, simulation was performed using three-dimensional computational fluid dynamics with state-of-the-art spray, turbulence and combustion models. The simulation was first validated against test data and then used for piston design optimization. The results show how the optimized torus combustion chamber design improves combustion and achieves balance between scavenging and combustion as well as piston thermal management.