What happens if you say, “No”?
It happens in court sometimes. If the witness is merely quibbling about the wording of the oath, the judge will usually work with him-- anything that says, in substance, 'I will testify truly and know I can go to jail if I deliberately lie' is legally sufficient.
If the witness flat out refuses to take any oath or affirmation, that's a refusal to testify, and the judge can hold the witness in contempt and have him jailed until he agrees to testify under oath.