In Optical See-Through (OST) displays, the chroma component in color space is often distorted. OST displays appear to have a uniquely different gamut feature compared with conventional color devices. In this paper, we proposed a novel two-step gamut mapping method for OST displays. The conventional CARISMA (Color Appearance Research for Interactive System Management and Application) algorithm is extended straightforwardly, and its process is divided into two separate steps (lightness mapping and chroma compression) by reflecting the characteristics of the newtype OST. We confirmed experimentally that the proposed gamut mapping method can reduce color distortion better than the existing CARISMA.