Thanks for your responses. I am not able to get confirmation from Delta as to when this change occurred, but they are now ticketing fares that originate abroad in the local currency. This is a change of their terms of service, and does impact anyone purchasing a ticket on delta.com with a card not located in the country of flight origin.

I feel a bit cynical that AmericanExpress and Delta have this tie-up with a credit card but are altering their practices to hand each other even more in fees from the customer. When I purchase a product from a US site with a US credit card I do not have a reasonable expectation of my payment being processed abroad. So while I understand the AmEx fee (My AmEx statement shows the tickets were processed in Oslo, hence the fee), I feel in this case the practice is dishonest as it was not called to particular attention (since it appears to be a new practice) and it costs the customer a significant amount in fees (an extra $80 may be cake compared to the fare price, but it is the principle that is at stake here), and benefits only their partner. If that makes sense.

if they are going to change their policy and it will impact their co-branded card users to such a degree, should they not note this loud and clear before the purchase is made, so the customer recognizes their financial details are being sent abroad and foreign transaction fees will apply? It would be different if it wasn't a Delta card, but surely they know how their policy changes impact their card carriers?

At any rate, Delta has not responded well, but AmEx has in this occasion.