As a life long Oregonian, I can answer.
You pull up to the pump, the attendant comes to your window, and you give them your card or cash, and tell them how much you want (usually filled, but I've asked for $20 before, especially when I pay with cash). They fill the tank, give you the receipt, and off you go.
It is illegal to pump your own, and you can be fined for it, although there is nothing about the pump stopping you, although the attendant will usually notice and run over to stop you pretty quickly. I believe that they might be liable for allowing people to pump their own.
As for tipping, I have never seen somebody tip the attendant, unless that attendant truly went out of their way to give good service. I guess you could, but it's definitely not expected.
There are a couple of exceptions. You can pump your own diesel, or if you have access to a commercial filling station like Pacific Pride.