HPB18 is the recommended battery for 18 volt Black and Decker slide battery pack power tools.
HPB18-OPE is the recommended battery for 18 volt Black and Decker slide battery pack outdoor products.
OPE stands for Outdoor Power Equipment. The HPB18 is 1.3 Ah and the HPB18-OPE is 1.5 Ah. HPB18 is recommended as the replacement for FireStorm 18 volt slide pack power tools. In plain English the OPE version has slightly longer run time. They are compatible with one another and interchangeable.