I am flying directly from Oakland, California to Oslo, Norway one one ticket. Will be flying within Europe on another ticket from Oslo to Frankfurt and then onto Milan. My question is will I have to go through customs (passport control) and then back through the security checkpoint before I can check in for my next flight? Or since I am still flying within Europe will I just have to do passport control and then can go on to my flight?
OSL has been my terminal point and I have not connected through OSL.
As the first entry point to a Schengen treaty country I would expect that you would have to go through passport control. If you can stay airside (e.g. not retrieve checked baggage) you would not have to go through customs. In any event, most visitors exit the nothing to declare door and customs is a non event.
If you're checking your luggage through to Milan, and are already checked in for your follow-on flight, you should not have to go back through security. (You shouldn't, but Norway is not part of the EU - but part of the Schengen Zone). If you have to touch your bags at all or change terminals, you definitely will have to go back through Security.