Its really easy to get around in Oslo because of the very good public tranportation systems. You can use buses, subway and trams. Every station has time tables. You can use all of them with the same ticket. You can buy one hour ticket, one day ticket, one week ticket or one month ticket if you are planning to stay in Oslo for a month.
One hour ticket costs 20 KR
One day ticket costs 40 KR (you can use the public transportation system as much as you want for 24 hours)
One week ticket cost 140 KR (you can use the public transportation system as much as you want for 1 week)
One month ticket cost 550 KR (you can use the public transportation system as much as you want for one month)