I booked a flight from Oslo to Helsinki and I want to cancel it! How can I do that, and will I have to pay extra charges?
Norwegian LowFare tickets for flights within the Nordic countries can be canceled without charge within 4 hours after the booking. After that, the tickets are non-refundable. If it's any help, you can however until 30 minutes before departure change the name, destination or time by paying a fee. Depending on whether or not you have further plans to fly, that may be a better solution that just leave the original ticket unused.
You can find all the change and cancellation regulations from Norwegian here.
The answer given by Tor-Einar Jarnbjo is correct, but I'll like to add some important, but little known information.
Regardless of reason for cancellation, you can get governmental taxes and charges reimbursed. No one gives away money for free and for fun, so you will have to fill out a hard-to-find-form, and pay a fee.