Open platforms become an increasingly popular phenomenon in industry practice and have proven to disrupt established industries within only a few years. Incumbent firms started to progressively adapt underlying business principles of open platforms, which are mainly known from successful tech-companies such as the likes of Google, Apple, Facebook, or Uber. By studying five incumbents from different industry settings, which started to transform their product focused businesses into open platforms, we derive interesting and counterintuitive insights for research and management practice. Our results uncover that open platforms of incumbents are formed around the companies’ existing infrastructure, do not exclude competitors to participate at the platforms’ ecosystems and make customers of the core business users of the open platform.