Tryvann is the main resort in the city of Oslo. It is a small resort with 10km of slopes. It is very easily accessible, since the subway goes from the city center up to the resort in just half an hour.
If you're going there for the runs, you will quickly be fed up, since it's small.
On the other hand, the resort has a good snowpark, for all levels and with 2 half pipes. You can spend hours in the park without ever getting fed up.
Conclusion: It's a good city resort, ideal for alpine ski/snowboard beginners and also for freestyle lovers.