I was very surprised at first to see that on every metro line there is a train only every 15 minutes. It seems like a very high time gap for a capital city metro. But when you get used to it and memorize the schedule, you can find that it is a reliable system. Trains are very rarely late. During the worse moths of pandemic people did not use the public transportation very much so the trains were not crowded but getting back to normal you can expect a much bigger crowd. I have not seen or experience any pickpockets so I must say the Oslo metro is a safe place, I know I don't have to always watch my backpack, whereas in other capitals you easily get robbed if you don't hold on tight to your belongings. The stations are regularly cleaned and the trains are only dirty because many uneducated people put their feet on the seats, sadly it seems to be a local specialty. Do take into consideration that the Norwegian culture is one of keeping the distance, so you should never sit next to anyone on the metro. If you sit down, then you know you directly have to occupy two seats. Note that most of the lines are not under the surface so it is not a traditional metro system (more like a train track) and the end of the lines usually take you directly into nature which is amazing. The stations in the city center are common for all 5 lines. There is frequent control on the trains so it is not worth not buying a ticket (fine is about 1000 kroner - almost 100 euros). The ticket machines have just been deactivated so your can now only buy your travel card online or at the shops that sell them (7 eleven, Narvesen etc.). If you buy a renewable period pass you still have to activate it every time the previous period expires, I find this very annoying to keep track of.