At Deichmanske Bibliotek (library) you can use a terminal for free up to 30 minutes.
At Unginfo in Møllergata use-it.unginfo.oslo.no/sider you can use a terminal for free up to 30 minutes. They are in theory only for people ages 26 or under. They will rarely ask you to prove your age, although if you are 50 and the place is crowded, you should rather head for the nearby library. During the summer, they also provide free tourist information targeted at budget travelers.
Most of the internet cafes are located in the eastern part of town, they are especially frequent in the Grønland part of town. Look for small shops selling telephone cards, they usually have internet terminals.
If you carry your own laptop, WLANs will be easy to find at cafes, hotels, bars and even in parks. For a good overview of free WLAN sites look here: no.freewlan.org/index.php/Oslo (in norwegian).
There are several open wifi connections on the upper level cafes at the train station Oslo Sentralstation, the subway station at Grønland. Inside the train station Oslo S On the third floor (one level up in the main hall) is the most complete Internet cafe - business center with full spectra of services and personnel onsite www.arcticinternet.com. On the first and second floor, there are several automated stand alone computers setup for basic internet surfing, as well as an Internet cafe in the bus terminal next to the central station.
Many 7-eleven shops have terminals you can use for a small fee, although this is not a very convenient way to surf if you need to use the computer for a long time.
Across from the Nationaltheatret next to a pub called Paddy's, there is an Internet café, and there are kiosks in the shopping center Oslo City in the downtown area and other wifi spots in the eastern fringes, such as the Deichmanske Bibliotek on Henrik Ibsens Gate.
Byråkrat www.byrakrat.no, QBA (Grünerløkka) and Cafe Tiger all have free open wireless internet. If you just want internet access, a cup of coffee for NOK 18 at Cafe Tiger is worth it.