It's just the games between Vålerenga and FC Lyn Oslo that you can really call rivalry derbies in Norway I think. The biggest cities in Norway are Oslo, Bergen, Trondheim, Stavanger and Tromsø but it's just in Oslo (the capital) there is two teams in the Tippeliga.

You have two more teams in the Oslo region, Lillestrøm SK and Stabæk. There is just Lillestrøm you can call rivals to the Oslo-teams. Maybe not to FC Lyn Oslo, but at least to Vålerenga. The matches between Vålerenga and Lillestrøm are usually quite open and entertaining with lots of fans in the stadium.

So, there is only one big city derby in Norway, and it is Vålerenga against FC Lyn Oslo. Come on Lyn!

(Sorry about my English.)