In spoken French, "on" replaces nous as a subject pronoun. I don't understand how to say something like
Do you want us to come?
You owe us money.
Do French speakers use nous here, even in highly informal conversation?
“On” can only replace “nous” when it is a subject pronoun. In the first example, the corresponding French sentence has “nous”/“on” as a subject of the verb come, so “on” can be used.
From more formal to less formal.
In the two other, “us” is not subject, thus using “on” is impossible.
Basically just remember that “on” is the equivalent of “one” (as in