Mechanical logic gates, whose embodiments were demonstrated in the past with rigid bodies connected with kinematic joints, are revisited in this work using compliant mechanisms. In particular, an OR gate is designed using pinned-pinned bistable arches. The compliant OR gate consists of five bistable arches arranged in such a way that a central arch acts as the output with 0 and 1 stable states while two pairs of arches, with their own 0 and 1 states, act as inputs. The arch-profiles of all arches are designed so that the forces of switching and switching back between the two stable states and the travel between the two stable configurations are as per the desired functionality. An analytical method, which is based on linear superposition of buckling mode shapes of a straight pinned-pinned column, was used to design the arch-profiles. Finite element analysis was used to validate the OR logic behaviour of the entire setup. A macroscale prototype was made using 3D printing to demonstrate the working of the OR logic. Since mechanical gates are useful at the micro-scale, using fixed-fixed arches instead of pinned-pinned arches is beneficial from the viewpoint of microfabrication. © 2021, Springer Nature Singapore Pte Ltd.