You will have to have your passport stamped at Keflavik... I'm not sure if you will need to in Oslo.
Customs in Europe = walk through the green "Nothing to Declare-Zone" - 2-5 seconds.
Customs in the US = waiting in line and taking ages for an actual person to ask if you have anything to declare = 1-60 minutes.
You will not clear customs in Iceland as you're an international transit passenger (something US customs, border security and TSA don't know exist) but will clear immigration as this is your entry point into the Schengen Area (basically an EU thing, although neither Iceland nor Norway are members of the EU - long story).
Thank you all!
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