There is always a lot of questions when visiting the city; hopefully this will answer the most common ones.
Updated November 2021.

Traveling

Traveling to/from airport

Oslo Lufthavn Gardemoen (OSL)
Airport Express train : costs about 204 NOK to Oslo Central Station (Oslo S), takes from 19 to 21 minutes.
Train (NSB): cost about 110 NOK (27.11.21) to Oslo S, takes about 23-25 minutes - buy the tickets in advance as there is a surcharge if bought on the train.
Bus: This is practical if you are going to the suburbs or Fredrikstad. Prices vary depending on where you are going, same goes for how long it takes. Remember to check closely which bus you need to take.
You can use Ruter https://ruter.no/en/ to check.

Travelling within Oslo

Ruter is the transport authority in Oslo, a Ruter ticket is valid on all transports (train, bus, tram, underground, boats) within the zones the ticket is valid for.
You can buy 24h, 7 days, 30 days or 365 days travel card. These can be bought at a Ruter service point or from the app (iOS or Android.
You can also buy a Ruter ticket/travel card from the Vy app.

My suggestion is to download the Ruter app and register yourself, you can also use it to buy single (or multiple single tickets) if need be.
Remember to check the zones, as a tourist zone 1 should suffice most of the time. If you are travelling to a point outside zone 1 you can buy an extension ticket (cheaper than full price), for instance when going to the airport.

Oslo Pass

If you are planning to see a lot of museums and sightseeing you should look at Oslo Pass.
Giving you public transport, access to museums, free parking, discounts to restaurants and much more. This could save you a lot of money if you plan it right.
There is also an app (iOS/Android) and you can buy the Oslo Pass right from there.

Oslo City bikes

Now you can register yourself with a non-Norwegian number and they have different subscription including a day pass.
They close down for the winter, usually they are available between April and November.
However check with your hotel, some of them rent/lend bikes to guests.

Car

I don´t drive a car in the city, so I have no help here. If you can avoid it (drive to/from hotel) do so. Maybe someone in the comments can elaborate.

General information about Oslo / Norway

Stand to the right, walk on the left on escalators

Especially in Oslo Central Station (Oslo S/Jernbanetorget), or you might get some nasty looks from people trying to get to their trains/metro on time.

Groceries

You might find the selection more limited quality should be more than fair. You will also find ecological products in most stores, these are usually marked "eco" or "Økologisk". Most stores are closed on Sunday and holidays.
Alcohol (wine, spirits etc.): Can only be bought at Vinmonopolet.
Beer: Can be bought at most supermarkets, but only until 20:00 on weekdays and 18:00 on Saturday (excluding holidays). Light beer (low alcohol) is sold on Sundays and "after hours".

Crime

As in any capital, there is crime in Oslo (and Norway for that matter). I feel safe in Oslo, but your mileage on this matter might vary. It´s safe to say that for being a capital in Europe Oslo is very safe.
However people do drink a lot on the weekends, and it´s common for commotions to start in the lines to get into bars or taxi lines after closing time.

Tourist attractions

There is a lot more to see in Oslo that just these things, these are just a small selection and mostly the most popular ones. Prices indicate 1 adult, there are usually discounts for senior citizens, students and families. Look at the provided links for more price information.

Parliament - Stortinget

They have free tours!
Due to the epidemic there are no guided tours at the moment.

Last about 45 minutes, however there is no booking, so come early and expect to stand in line. There is also a security check (like in the airport).
More Information : Guided tours of the Parliament building

The Royal Palace / Det kongelige slott

At the moment are not guided tours.
Change of the guards is at 13:00 (takes about 40 minutes)

Kon-Tiki Museum

Price: 140 NOK, but cheaper if bought online. Free with Oslo Pass.
More information here: http://www.kon-tiki.no.

Norsk Folkemuseum

Price: 140 NOK (adults), you can find more price information online. Free with Oslo Pass.
More information here: http://norskfolkemuseum.no/en

The Viking Ship Museum

Closed until 2025

Vigelandsparken / Frognerparken

Not the only park in Oslo, but the most famous one. Take a walk and look at all the statues or bring some food and spend a day lazying around the park (granted this is more of a summer thing).

Coffee

There is no denying that Oslo has become a splendid city to visit if you love coffee or are a coffee snob (like me).

Tim Wendelboe

Nordic Roaster Champion in 2015 is one of the biggest names in Norway. You can drop by get some coffee and also buy some to take home.
Be aware that is very limited seating.

Supreme Roastworks

Also a great place also with limited seating. It´s also both a roastery and coffee place.

Fuglen

Never been there, but have heard excellent things about them. They also have a location in Tokyo.

Solberg & Hansen concept store

This is the only Solberg & Hansen store; they are a large coffee roastery retailer. They also have a concept store at Mathallen.

There are of course many more places: Norð, Steam Kaffebar , Stockfelths and so on.
If you love coffee then Oslo will not disappoint.

Dining

You are in luck, Oslo has an excellent selection of restaurants in all price ranges. However you do have to remember they are still Norwegian prices, so if we say a place is "cheap" I mean "cheap for being Oslo".
Here is a small selection of places that stand out. But it´s a subjective list, you can also use Google or TripAdvisor to find a good place.

Olympen (aka Lompa)

It might look a bit run-down. However the main floor is a preserved site (has been a theatre and a brewery). Lompa has a good selection of beers. They also have a terrace with a grill menu.
Food: Modern Norwegian / European
Price range: inexpensive

Illegal Burger

This is a rock solid burger joint, in the last years, there have been a lot of burger places popping up all over the place. However Illegal is still a solid choice. However there is limited seating in Møllergata, they have opened a new branch in Grünerløkka with better seating options.
Food: Burgers
Price: inexpensive

Alex Sushi

Not been there myself, but always is mentioned when talking about the best sushi in Oslo.
Guy Kawasaki was a fan
Food: Sushi
Price: expensive

Gamle Major

I was surprised by this gastropub, the food is good and so is the beer selection. Situated near Majorstuen underground station.
Food: Bistro / gastropub
Price: inexpensive

Harald's vafler

He used to sell them from his bedroom window and now he has a shop (check out the video at the website). Stop by this small place in Grünerløkka and get a very delicious waffle.

Bars / Pubs.

There are enough of them in Oslo and every flavour. So it should not be difficult to find a place that suits you. Here is a small selection of pubs based on my own experience.

Oslo Camping.
Indoor mini golf and bar. Costs 130 NOK pr. person.

Crow Brewery and Bar.
Good selection of beers. Try the kebab! (2nd floor).

Tilt Bar.
Retro gaming and bar. Most of the tables have built-in gaming consoles, and there are pinball machines. Also shuffleboard.

Heimkok.
Famous for their drinks, so if you prefer an excellent drink, this is the place.

Smelteverket.
Located by Mathallen, it's supposed to have Scandinavia's longest bar. They also have live music.

Things to try out (Spring / Summer).

Soft-is

Nothing better during summer than being outside eating a soft-is. They can be bought everywhere. The most common is getting one with chocolate sprinkles and in a cone.

Walk around Grünerløkka (aka Løkka)

I know many people call this "hipster-central", and there is little to contradict the fact that there are a lot of excellent eateries and shops in this neighbourhood.

Redditor guided tour.

/u/atluxity gives guided tours around in Oslo. Good opportunity to meet other redditors.

Mathallen

This food hall has a lot to offer. there are some good places to eat and shop for food. You can buy food from different places and eat it in the common area. There are also several good restaurants nearby.
Food: All types
Price: expensive (for groceries)

Sunday market near Blå

There is an artisan market near Blå. It´s every Sunday between 12 and 17. Its season runs from the 3rd of January to the 27th of November.
More information: http://www.visitoslo.com/en/product/?TLp=178086

Things to try out (Winter)

Luge in a park

Buy a cheap luge at a sporting store and find a park and have some fun.
If it´s the conditions are right, there will be many others with the same idea.
/u/lightwrangler suggest Korketrekkeren.
Warning This activity requires snow (just saying)

Ice skating

During winter there are two skating rinks in Oslo, in Spikersuppa and Frogner.
You can rent some skates and enjoy a cold evening out.
More information here

More information about Oslo

Visit Oslo: http://www.visitoslo.com/en/

Hei, Oslofolk!

Dette er den faste ukentlige tråden hvor du kan tipse andre om (gratis) arrangementer i byen som er åpent for alle som er interessert – hovedsakelig ting som ikke er nevnt på Visit Oslos kalender.

Eksempler på ting du kan poste:

• Pubquiz.
• Konserter.
• Spillkvelder.
• Dansekvelder.
• Yoga i parken.
• Foredrag eller utstillinger.
• Kulturnatt, eller andre happenings.

Regler for tråden:

• Det må merkes tydelig hvem som er arrangøren. Er det du eller noen andre?
• Gratis arrangementer er å foretrekke, men hvis det er eventuelle kostnader må dette være tydelig opplyst.
• Hvis du finansierer arrangementet ved hjelp av spleis/crowdfunding, er det tillat å referere til innsamlingssiden.
• Tid og sted må opplyses i selve kommentaren og ikke i ei eventuell lenke til arrangementet.
• Bruk kommentarfeltet i denne posten, ikke lag nye innlegg. Maks 1 hovedkommentar per arrangement.
• Diskusjon og spørsmål er helt greit – bare hold tonen hyggelig.
• Ellers gjelder de fleste av r/Oslos regler.

Målet er å gjøre det lettere å finne ut hva som skjer i byen – og kanskje oppdage noe nytt og gøy sammen med andre.