In this paper, we propose a novel block cipher mode of operation (BCMO for short), named Output Protection Chain (OPC for short), which as a symmetric encryption structure is different from other existing BCMOs in that it employs two keys, rather than one key, to protect the output of the mode. The security threats of chosen-plaintext attacks on three existing common BCMOs, including the Cipher Feedback mode (CFB), the Output Feedback mode (OFB), and the Counter mode (CTR), are also analyzed. After that, we explain why the OPC mode (or simply the OPC) can effectively avoid chosen-plaintext attacks, and why its security level is higher than those of CFB, OFB, and CTR.