When camera and minimOSD are connected to video transmitter, minimOSD is not overlaying OSD data. If camera is disconnected, OSD data is transmitted. After connecting camera, only camera image is transmitted.
Check if camera and minimOSD are using the same display system. Usually monitor/goggles and video transmitter does not care very much about used display system. Transmitter just transmits, and monitor/goggles very often can detect if signal is PAL or NTSC. But if camera is sending PAL signal, minimOSD has be overlaying imag using PAL subroutine. If it will try to use NTSC mode, it will fail. In case on MW OSD software (Scarab OSD), OSD display system is configured in GUI. Set it to the same system as camera.