notiflyr

Google flights thinks it's more, but pricable on Ex & PL it seems.
Found AA/DL/AF/AY/BA/AA/IB/FI/WW itins

MIA-CPH $359

scibuff

All dates I tried show over $3k for ow. Kinda useless to post this if you're not gonna give a direct link to an example (or at least say the travel dates)

Watermaniak

It looks like OP has some sort of Multi-city trip configured that we don't know about (yet)

notiflyr

it's RT not one way
Looks to be available through may and then after summer..
availability is drying up though.

ORbitz $389
Orbitz $359

Google flights shows this at a higher rate, so you can use ITA or Google flights to find dates. orbitz & expedia should price this at the lower rate.

xliioper

Seasonal Basic Economy fares to HEL/OSL/CPH/ARN, etc. 7-day minimum stay required, so not too useful as a MR. JFK-OSL on DL as low as $322 RT (fare filing VK7L87B2). Also available out of CHI and SFO in addition to JFK and BOS.
DAY/TIME APPLICATION
OUTBOUND - PERMITTED SUN THROUGH WED ON EACH TRANSATLANTIC SECTOR. INBOUND - PERMITTED MON THROUGH FRI ON EACH TRANSATLANTIC SECTOR.
SEASONALITY
OUTBOUND - PERMITTED 01APR 19 THROUGH 11MAY 19 OR 09SEP 19 THROUGH 29SEP 19 FOR EACH TRANSATLANTIC SECTOR. INBOUND - PERMITTED 01APR 19 THROUGH 21MAY 19 OR 07SEP 19 THROUGH 28OCT 19 OR 25DEC 19 THROUGH 07JAN 20 FOR EACH TRANSATLANTIC SECTOR.
ADVANCE RESERVATIONS AND TICKETING
RESERVATIONS FOR ALL SECTORS ARE REQUIRED AT LEAST 28 DAYS BEFORE DEPARTURE. TICKETING MUST BE COMPLETED WITHIN 72 HOURS AFTER RESERVATIONS ARE MADE OR AT LEAST 28 DAYS BEFORE DEPARTURE WHICHEVER IS EARLIER.
MINIMUM STAY
TRAVEL FROM INBOUND TRANSATLANTIC SECTOR MUST COMMENCE NO EARLIER THAN 7 DAYS AFTER DEPARTURE OF THE OUTBOUND TRANSATLANTIC SECTOR.

notiflyr

FLL just joined the party.. about $50 more though