Is it considered an innovation (bid‘ah) for worshippers to repeat “O Allah” after the Imam when he supplicates, and was this reported from the Messenger of Allah, may Allah’s prayers and peace be upon him?

If the Imam supplicates aloud in Qunut, the Sunnah is for the congregant (ma'mum) to say "Ameen." However, for the glorification that punctuates the supplication (such as: "Indeed, he whom You have befriended shall not be abased"), it has not been established that the congregant should say "Ya Allah" or "Nashhad" or "Haqqan." Rather, one should say "Ameen" or remain silent. Some jurists have gone so far as to invalidate the prayer of one who says "Ya Allah" and the like, and some scholars have considered it an innovation (bid'ah). Some jurists have recommended that the congregant participate with the Imam in glorification silently. As for saying "Ya Allah" instead of "Ameen" in the place of supplication, it is an innovation.