This paper presents a public-key-based optical image cryptosystem with adaptive steganography for practical secure communications. The optical image cryptosystem employs a hybrid architecture for ciphering and deciphering in which double random-phase encoding and asymmetric encryption algorithms are utilized for images and session keys, respectively. The session key is safely protected and transmitted by using an asymmetric encryption algorithm and an adaptive steganographic scheme, respectively. To perform the adaptive steganographic scheme, a sorting technique is used to find the suitable embedding position in the embedding domain, a least-significant-bit truncation algorithm is presented to find the invariant hiding order, and a quantization-based data-embedding algorithm is utilized to hide message bits. Experimental results show that the proposed scheme is superior to that of a previous one due to Lin et al., no matter what embedding domain, quantization level, and message size are used. Especially, compared with the latter scheme a large improvement (22.56 dB) of image quality is achieved by using the proposed adaptive steganographic scheme.