is it OW-ch or OH-ch?

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That fuzzy, great tasting creature chewing the grass in the field is a cow, pronounced c-OW. When dog barks, we claim its sound is bow wow, pronounced b-OW w-OW. Yet, if you take a piece of cloth and put it around your neck, it becomes a bow tie, pronounced b-OH. What changed? What made the OW become OH? If we string bunch of dogs together, do we get bow ties or bow ties?