Not all, but some USPS locations do require an appointment. And for those that do, next available appt. may be months away, so it's always best to check ahead. You can call them directly or you can check Travel.state.gov website and navigate to "Where to apply". After entering your zip code, all nearby locations will be displayed where you can see whether or not they require an appointment.
It all depends on whether Newark NJ office offers same day oath ceremonies or not. After the interview you will be informed about taking the oath. Don't hesitate to ask IO any questions you may have about it. If your interview is early in the morning, you may be offered same day oath. If not, most likely you'll receive your oath letter with a ceremony date and location, right after the interview,(in about an hour or so). If for whatever reason you can't wait for your oath letter on the day of your interview, it will be mailed to you, although I would strongly suggest you wait for it, just in case the oath is scheduled sooner than the mail can get to you. Good luck
another1 got a reaction from JimmyHou in US passport after Oath Ceremony: send application or apply in person?
another1 got a reaction from squareleg in N-400 December 2015 Filers