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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?

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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:

We can buy a new house as soon as we sell our old one.

Here, "as soon as" implies that it will happen almost immediately after.

Once we sell our old house we can buy a new one.

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:

Please do this as soon as possible.

This is an instruction to do something at the first available opportunity.

Please do this once possible.

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:

Please do this at once.

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:

Please do this once.

Which would mean do this only one time.

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The have slightly different semantics. As soon as implies that there is no wait between the cause and the effect. Meanwhile, once only implies that the effect will happen some time soon after the cause. Furthermore, as soon as also emphasizes the lack of delay.

For example, a case where you can only use once would be:

Once we receive in your application, you will get a confirmation in the post.

Here, as soon as wouldn't be applicable because there is probably a 1+ day delay between them receiving your application and the confirmation arriving at your house.

As soon as he heard the news, his gut dropped.

Once would might be technically applicable here, but as soon as is preferred because there would likely be no delay between the man hearing the news and his gut dropping.

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