There are several possibilities for swimming in and around Oslo, and when the weather is nice and warm, the beaches are crowded with people who enjoy that lovely summer feeling of relaxing on the beach and going for a dip.
Even though I have grown up by the sea and really prefer to swim in saltwater, my favorite place to be in Oslo on a warm summer day is a grass lawn next to a river or a lake.
Frysja is a recreational area at the upper end of Akerselva. The river runs from Maridalsvannet, which is the drinking water source of Oslo, and all the way through the city into the Oslofjord. Close to the Technical Museum, there are large grass lawns on both sides of the river, and it is a lovely place to spend the day. People go there for swimming, barbecues and picnics, and it is a popular place for families as well as youth.
It is easy to get there from town, take bus number 54 towards Kjelsås. There is a grocery store just next to the bus stop at Frysja to buy refreshments and snacks.
It is also nice to walk the 8 km long trail along the river from town; you should make sure you pass by my other favorite part of the river – the city waterfall Møllefossen (check the Møllefossen article).