This will be my first time out of the U.S. so it is unclear to me. Apologies if this is obvious. The flight I am looking at will be from Oakland to Oslo and then to Berlin on Norwegian Air. There is a 1.5-hour layover in Oslo. I don't know what the procedure for a layover like this is (immigration, customs?) and I want to make sure that this is enough time to make the connection. What do you think? It is worth mentioning that I travel lightly and will probably not check a bag.
As Gagravarr said, if it's on the same ticket, you don't need to worry. 1.5 hours is plenty of time even if you have to go through customs and have a checked-in baggage. Oslo airport is pretty small, so even if your first flight arrives slightly later, you'll still make it easily. In case it's late more than an hour and the flight to Germany is on time, then you're very likely to miss that one, but if it's on the same ticket, the airline will deliver you to your destination as soon as possible with no extra charge. They have three direct flights per day from Oslo to Berlin and many more with a connection elsewhere, so you shouldn't worry too much. In case you're planning to buy two separate tickets, then it's a big no-no. Unless you have a very good travel insurance covering all those expenses, you should not be taking a risk like that, because in that occasion you're responsible for the flights you missed after the airline fulfills their duty of delivering you to Oslo with that first ticket you bought.
Norwegian Air's web site has an explanation on connections.
If you have a single ticket with a connection, ninety minutes is more than enough time for the physical transfer and, moreover, if you miss the connection because of bad weather or another delay, they will book you on their next flight and pay your accommodations meantime.
For two separate tickets, one to Oslo and another from Oslo to Berlin, they disclaim responsibility for connections under two hours, so that would be at your own risk for delay. But if the flight is on time or close, you would make your second flight easily.