When you say "onset", it makes me think of the onset of a disease or condition or some other thing that you don't really want. (The OED says "The beginning of some (esp. unpleasant) situation, condition, or state".) Examples that I have found on the internet include "onset of winter", "onset of a cold", "onset of puberty". The "outset" is more like the beginning of an event, like you're setting out on an adventure. (The OED says "The action or fact of setting out upon a journey, course of action, business, etc.".) Th
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