This beach gets really packed during the summer! There's also an adjacent stretch where clothing is optional.
More information http://www.visitoslo.com/en/activities-and-attractions/boroughs/bygdoy/activities/?TLp=181567&Huk--Paradisbukta-beach
Address 0286 Oslo, Norway
Coordinates 59°53'41.875" N 10°40'34.711" E