Abstract: RF jamming poses a severe threat to 5G/6G networks, increasing packet latency being especially disruption to mission-critical URLLC services. This paper introduces a proactive Jamming Detection xApp (JD-xApp) for the Open RAN architecture that detects attacks by monitoring the moving-average Block Error Rate (BLER) via the E2 interface. Upon detection, the algorithm overrides standard link adaptation, enforcing a robust upper ceiling on the Modulation and Coding Scheme (MCS) to stabilize latency. To counter O-RAN platform adoption challenges, we transition the framework to an open-source Centralized Unit/Distributed Unit implementation (OCUDU). Evaluated with high-end Keysight lab equipment including the UXM 5G Wireless Test Platform and the PROPSIM F64 channel emulator, the JD-xApp reduces the expected number of packet retransmission attempts by about 67.3%. Finally, the system's feasibility is validated via over-the-air deployment using the POWDER lab infrastructure.