Technology advancements are underpinning firms in shaping their products and services into digital platforms to foster value co-creation in their platform-based ecosystems. While existing research has mainly focused on business-to-customer (B2C) platforms, relatively little research has been conducted on business-to-business (B2B) platforms. To address this research gap, this study employs a case study approach to collect and examine three out-bound open innovation (OI) application cases in the context of B2B platforms, namely, TSMC, IBM and CNT Tech. The case analysis results show that the coverage of B2B markets can be expanded and diversified by OI. To improve the quality of platform offerings (not only platform services but also complementary innovations), the case firms implemented OI applications comprising two phases that manage knowledge outflows (with boundary resources) and inflows (with input/output controls) across their organisational boundaries. Knowledge sharing provided a B2B platform owner new market creation opportunities, and complementors combined and pivoted some of the platform owners’ core technologies, consequently diversifying the platforms’ applications and making platform ecosystem more dynamic and vibrant.