Getting drunk in Norway is not as easy as in Spain or even USA. Here's a handy little guide to the wonders of consuming alcohol when in Norway.
What to get
Norway is a country with beer traditions going back thousands of years. The climate is cold and not very suitable for wine. Hence, the main alcoholic drink is beer, wine is usually used in combination with a meal. Norway is also the home of the Aquavit, which you'll find in almost every home in the Christmas season.
All strong alcohol and wine is sold through a system of stores called Vinmonopolet. This is a widely hated system, but it is also the store with most alcoholic brands in the world (5000 different brands. Yowza!). Prices are high. This is due to the incredible high taxes on alcohol, a full bottle of vodka will set you back some 35 USD. A bottle of young French wine costs around 10 USD. Yeah, it's expensive.
The upside is that you can legally buy beer and wine from the age of 18 and strong alcohol from the age of 20. And the service is excellent. You won't get better advice on what wine to get in any other store. The clerks know their business. These stores are open from 09:00 to 18:00 on weekdays and 10:00 to 14:00 on Saturdays. All stores that sell alcohol are closed the day before a christian holiday.
Unfortunately, it's up to the different counties to decide when the sale of beer in the grocery store closes. Oslo closes at 20:00 on weekdays and 18:00 on Saturday. If you haven't paid for the beer before the closing time, you're fucked. Some immigrant stores will sell you the beer, illegally. There is no risk in when buying from these stores, only the store owner can be punished for breaking this law.
The different counties can also decide to not allow sale of beer in stores, or to limit the sale to specific stores. This is decided by a referendum in the different counties and does not happen in the cities, only very small rural districts.
Update I: There was a story on the news, and there is actually only one county left not selling beer in their stores. That county is Aunedal, and they are having a referendum on the beer sale now.
Update II: Allright! The result from the refrendum: You can now buy beer in all counties in Norway. Yay!
You might also stumble over something called "Heimbrent" (home distilled). This is grain alcohol, very strong. My advice is to NOT, I repeat, NOT drink this. The police will bother you if you are caught with this in the larger cities.
Update III:OK. We had a MAJOR scandal with methanol here in Norway. Somehow, the booze from smugglers has been contaminated with methanol. The methanol content is up to 20%, more than enough to slowly kill a person. This is one of the reasons you shoyld NOT drink ANYTHING you haven't bought legally at Vinmonopolet. Vinmonopolet boosted their sales after the scandal became known.
Vorspiel and nachspiel
Most young folks don't have a lot of money, so they buy beer in the stores and have a little party at home before hitting the town. This is called "vorspiel" (German for foreplay), and is both cosy and practical. Just don't get so drunk that you'll be denied at the bar when arriving.
The nachspiel (German for night play) starts when the bars close. Located at a private place, this is where you get to know other persons and raid their liquor cabinet all at once.
Oslo, the capital, is a place with many bars given the size of the city. Most nightclubs and bars open at 21:00. Hotel bars are open from 16:00 or all day long. The serving of alcohol in the city is limited to 02:30 in the centre of the town and 01:00 in the areas where people live. The bars and nightclubs can remain open longer, but they can't sell alcohol ergo they close shortly after the limit. All bars are closed on and the day before christian holidays. They are also closed on Sunday.
Serving of beer is age limited to 18 years or above, to buy liquor (13.99% alcohol and above) you have to be 21 or older. Bars that serve liquor have to set the limit to 21 years. Bars that only serve beer and alcopop have a younger crowd and are less attractive to an adult audience. A lot of young people take the chance of getting a fake ID, so the median age will be about 17 in those younger bars.
There is a local city authority that monitors these rules and can order a place closed on the minute, for up to three months (if you go to a bar and it is being "redecorated" for two weeks or more, they've been caught). The authority looks for underage people, so the bouncers are very strict with ID in the doorway. A passport will always do, American drivers licenses are not valid. Do NOT argue with the bouncers, they are not rational people and often high on different drugs. If there's a fight or incident, the police shut down the place.
The bartender is not allowed to serve you more than 6 cl of alcohol in one drink, so you can get a long island ice tea without any problem. But not a double. If you order some strange drink that contains more than 6 cl of alcohol, just use the Utah-way of buying alcohol, make it two different orders and mix it yourself. Easy and entertaining.
The quality of the beer is very different. The bars with a younger crowd will normally not focus on the quality of the beer and some cheat by watering it down. They also don't know how to handle the tap system and glasses, making the beer experience something you could do without. Fortunately, there are some microbreweries that make excellent beer.
You are not allowed to take alcohol out of a bar, or to bring alcohol in. The bars and restaurants are not allowed to sell you unopened bottles to bring home.
The current but soon not trendy place to get drunk in Oslo is Grünerløkka. The bars are filled with neo-yuppies, and the music is soft. Good drinks.
Be aware that since the bars close at the same time, getting a cab can be very difficult around closing time. Fights over cabs are not uncommon, and the lines at the cab stops can be long. As matsmats states, try to catch the cabs on the flip side, on their way back to town from a fare. If you are in Oslo, the main roads inwards the city centre is: Ullevålsveien, Drammensveien and the E18. Your best bet for getting a cab would be at the Opera hotel at the railway stating. Your worst bet would be the cab stop at Stortinget. Also beware that the cabs aren't allowed to pick up passengeres if you aren't within 100 metes of a cab stop. Many don't quit at closing time, if you are invited to a nachspiel, join in. It's fun and you get to meet other people. And, if you are lucky, shag them.
The police will not disturb you when drinking, if you act in a civil matter. But you are not allowed to be wasted in public. The police are also strict about not letting you drink alcohol in public during the peak drunkness hours in the evening. Having a beer in a public park is "allowed" as long as you aren't out of order.
If you collapse on the street, someone will probably try to make contact with you. However, in the winter there's a good chance that you'll freeze to death before someone makes contact with you so don't get that wasted if you have a long walk home. This goes double for rural areas.
If someone can't make contact with you, they'll call the police. It takes the police some 10-15 minutes to arrive on a non-distress call like this, the person that called might or might not be nearby to guide the police. The police will try to talk to you, and if you are coherent, they'll try to find out where you live. If you manage to say where you live, they'll search you for money to a cab and get you one. If you don't have the money for a cab, the police will either drive you to the drunk tank or home depending on the amount of work that specific evening. If they can't get an address from you, they'll take you to the drunk tank. Usually, you won't get a fine if you have not done anything illegal. But you can. The police might also take you to the ER if you have to be pumped. Make sure you have a traveller's insurance before coming to this country. Medical care is free for inhabitants, but definitively not for foreigners.
Be aware of the city of Tønsberg. The city has astronomic penalties for peeing in public (approx. 1100 USD) and fighting (approx 2200 USD). They also have a no-tolerance policy.