Oslo has a lot of green spaces, but this one is one of my favourites. It is located on a hill, so in the winter it is the perfect place to catch some rays of sun on the darkest days. But it is even better in the summer, when it fills up with locals who make the most out of every chance to get outside.
This is how you enjoy a summer evening at St. Hanshaugen the local way: go to a supermarket to buy some drinks and snacks and maybe a BBQ and some food to cook on it. Alternatively, you can pick up sandwiches or take away dinner from one of the many places located near the park. Then you climb the hill until you get almost to the top. Bring a blanket, and find yourself a spot in between all the other people. Sit down, relax, people-watch and just enjoy the atmosphere, the views and the long light summer evening.
PS: It is not technically allowed to drink alcohol in public in Oslo and you may be fined, so I cannot officially recommend that. But behave yourself and follow the example of those around you, you will probably be just fine!