As I see it, there are at least four possible interpretations:
1. It was gasped by Mary Kelly during the attack.
2. It came from someone who discovered the murder.
3. Someone said it, but it was completely unrelated to the case.
4. Prater's account can be chalked up to an instance of suggestibility. Having found out that a murder had occurred the night before, Prater's memory was primed. The night before, something startled her and the cat at 4 am. But by the morning (and the info), she quite unintentionally recollected some other noise as "Oh, Murder!" That is, her mind played a trick on her.
As previously stated by others (including myself), "Oh, Murder!" is an odd thing to say upon being attacked by Jack the Ripper. On the other hand, discoverers of previous victims (Nicholls, Chapman, Stride) immediately went searching for the police. If #2 is true, why did the person not come forward. #3 is problematic to me because it just seems like too much of a coincidence. In the end, I think it is either #1 or #4. What do others think?
I think it was MJK who said "Oh, Murder" i also think she was asleep when he attacked her and awoke to a figure looming over her. I also agree that it seems unlikely the call came from someone in the vicinity. If that was so, why did they not go to the police?