If you check in at the airport you will need to either swipe your passport through a kiosk or show it to a ticket agent at the check-in desk before you can get a boarding pass. This is so the airline has your information, knows it is valid, and does not get stuck flying you home (and fined) if they transport you without a valid passport.

My experience is that if you do OLCI and print a boarding pass at home you can proceed directly through security. You should not have a problem clearing security with a DL or other acceptable non-passport ID. Just don't show them the connecting boarding pass heading out of the country to avoid them trying to make the "big catch" of an international pax without a passport.

You will have to show your passport at the gate on your international segment. If you did OLCI there will be something printed on the BP indicating that the airline needs to verify your passport, so they will do this at the gate. Worst case is they take care of it right there by swiping it or typing some stuff into the computer.