In Oslo, shops are usually open from 10 am - 5 pm and on Saturdays from 9 am - 2 pm. There are extended opening hours in most shopping centers 10 am - 8 pm (Mon - Fri) and on Saturdays from 10 am - 6 pm.
Extended shopping hours are not as popular in Norway. Most shops are closed on Sundays, but some souvenir shops stay open. Thursday offers late night shopping: shopping centers and souvenir shops generally offer extended opening hours until 7 pm or 8 pm on that day.
Oh, and you may need some cash, so keep in mind that most banks are open until 5 pm but have a 24-hour cash point (ATM) on the outside of the bank.
Byporten Shopping is Oslo a relatively new shopping center and is right next to Oslo Central Station (Oslo S). It includes right around 70 shops, even a Scandic Hotel, Norway's largest Egon Restaurant (among 11 other food places), as well as underground car parking. And the nice thing is, it's right next to Oslo Central Station. If you are changing trains and have a couple of hours in between transfers, hop on over here to Byporten and have a meal or a look around. You'll find all kinds of price ranges here. This shopping center is open 10 am - 9 pm on weekdays, and 10 am - 6 pm on Saturdays.
Oslo City Shopping Center
Built by Selmer Skanska in 1988, Oslo City Shopping Center is Oslo's largest and most popular shopping center. About 16 million people come here every year, and so many people can't be wrong. The selection is staggering. The shopping center currently has approximately 93 shops and restaurants. It was chosen as the best Nordic Mall 2010 even. This shopping center is centrally located within walking distance to the central station. During the warmer months, fresh groceries can be found at the entrance. Bad news? It can get very crowded here, and not just in the month before Christmas - and the bathrooms aren't free, either.
Karl Johans Gate Shopping Area
Karl Johans gate is Oslo's most famous pedestrian street and it's right in the center of Oslo. This street runs east to west from Oslo Central Station to the Royal Palace. Here you will find several street entertainers, restaurants and not to mention countless shops, including fashion chains such as Benetton and H&M. Prices are reasonable considering the location, and the easy access to open air is also nice. It doesn't get crowded too much, either. This street (and its back streets), is in particular famous for handicrafts, clothing, jewelry and look out for the home accessories within department stores. It's a must for shopping fans!
The Paleet Shopping Center
The Paleet is situated right by Karl Johans Gate, complementing the pedestrian shopping street we noted above. Paleet alone offers around 45 shops and 13 restaurants. It's a bit more upscale here, not exactly suitable for bargain-basement-shoppers. Expect to find ladies' fashion, men's fashion, porcelain, flowers, glassware, jewelry, and sportswear etc. at higher-end prices. Open on weekdays from 10 am - 8 pm and 10 am - 6 pm on Saturdays.