Signal come out of the cam and into the OSD and down the OSD's grd. trough the Rx and trough the BEC to finally reach the batts. Problem is your OSD's ground is long and also carry the servo's current. The ground level (0V) of your OSD is not exactly 0V, but change slightly as current from the servos pass. Solution: Feed your servos directly from the Rx, only the signal reaching the OSD.