I feel like they can be used interchangeably like the following sentences:
As soon as she saw her boyfriend, she felt so happy.
Once she saw her boyfriend, she felt so happy.
As soon as you do this, you will succeed.
Once you do this, you will succeed.
I made those examples, so there might be some other mistakes. If they are exactly the same, which one is more common or colloquial in the USA?
Best Answer
In your examples, yes they are interchangeable.
Essentially both sentences state that something happened (she felt happy) once a condition was met (she saw her boyfriend).
However in other situations the two may carry slightly different inferences. There are also other situations where they are not.
Example of different inferences:
Here, "as soon as" implies that it will happen almost immediately after.
Here there is not quite the same implication of immediacy. The condition is there, but it is not explicit that buying the new house will happen immediately.
An example where they are not interchangeable:
This is an instruction to do something at the first available opportunity.
A native English speaker just would not say this. Even if phrased as "please do this once it is possible", it does not sound natural.
You'd more likely hear:
This would mean do something immediately. To be clear, "at once" does not mean the same as "once" in your original example.
And of course there is another entirely different meaning of "once" which would be completely confusing if used:
Which would mean do this only one time.