Mini banks (the Norwegian name for ATMs) can be found adjacent to many banks and near busy public places, like shopping centres and central train stations. You can withdraw cash using major credit cards or Cirrus, Visa Electron or Plus debit cards with a 4-digit PIN. Banking fees for ATM withdrawals and foreign transactions vary widely, so please check the rates with your bank.
In general, the most known cards as Visa, MasterCard, Eurocard, Diners Club and American Express are widely accepted throughout Norway. Also things like buying train tickets or make purchases on some (but not all) domestic ferries and Hurtigruten cruises. Not all places accept debit cards.
Although least common, traveller's cheques are also accepted at banks and major hotels or tourist centres. The exchange fees vary by location.