My daughter will actually be 3 in August, and she goes through these phases of saying "oh no" constantly. Even if nothing appears to be wrong, to an adult at least. Sometimes she just says "oh no" and sometimes she'll point to something and say it. I try not to make a big deal about it, and tell her things like, "oh it's okay" or "that's no big deal". Every once in awhile it will get under my skin though, and after hearing "oh no" for about the 100th time in a day I'm like okay, "stop with the "oh nos" now". She never does though... *Sigh*, she never does.