Yes, it does sound like, as Clare said, he was either in pain or has associated the harness with something frightening. I had my old black cat who loved a little bit of cheese. We had a carpenter in one day and he started drilling when Harry went to eat the cheese. Harry never ate cheese again as he associated the noise of a hammer drill with his cheese. Strange but true! If you take Oscar (cool name by the way
) to get checked at the vets for anything like arthritis etc which could cause pain? Also had you got a little bit of his fur caught in the clips? Just throwing out ideas! You could probably go through a desensitising programme with him and the harness ie feed/play/cuddles/treats when the harness is on to get his used to it again. Short times with it on and building up to longer. Maybe he needs to wear it round the house for a good while before venturing out again. Sorry if im stating the obvious. Good luck to you and Oscar and hope he is soon enjoying his walks again