RAN openness is a vision for 5G and beyond to avoid unnecessary lock-in effects. In this regard, the O-RAN alliance provides a new architecture with RAN intelligent controller (RIC) for both non-real-time and near-real-time cases, together with running applications, i.e., rApps and xApps. However, two key challenges remain in the current xApp framework: platform lock-in and xApp reusability. Therefore, we introduce a novel FlexApp framework to address both issues, as well as a new E2* interface that consumes less latency and CPU utilization. This new interface speeds up the xApp development process. Our performance evaluation of the FlexApp prototype shows that it can realize scalability and ultra-low latency operations (<10 ms), as well as the capability of two-level abstraction for xApp development.