There is a difference between עד and עונה (witness): One is called an עונה from the perspective that he gives testimony in court, and it comes before the prefix ב. And one is called an עד from the perspective that he is designated and is valid as a witness and from the perspective that he warned the accused. And it only means that he is valid as a witness, and he is called an עד even if he does not give testimony before the court.