Sorenga Seawater Pool is Oslo's most accessible fjord swim, a public bathing area on the new waterfront east of the Opera House. It has a seawater pool, children's pool, small beach, lawns, and broad timber decks that fill quickly on warm summer days.
Even if you do not swim, it is worth the walk for the views back toward Bjørvika and the islands of the Oslofjord. Bring a towel, expect crowds in good weather, and combine it with the Opera House roof or the Munch Museum nearby.