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Will an American passport agency issue a passport with just a one way ticket?
Yes. To get a passport using the in-person service, you need to show proof that you are traveling internationally in the next 72 hours. (If your trip requires a visa, you can use the in-person service up to 10 days before your trip.) A one-way ticket is just as much proof of that as is a round-trip ticket.
Details are available at https://travel.state.gov/content/travel/en/passports/get-fast.html.