If you want to check out two "legendary" skate spots in Oslo, ask for direction to Rådhusplassen and Tinghuset. These two spots where mostly used in the 90's and early 2000's, but are still in use these days, I think. Rådhuset includes one small staircase and a huge one with a huge handrail. Tinghuset includes two ledges. Both places are great for flat skating.
Rådhuset is currently under construction, so it's not good at the moment. Tinghuset is great, but it's adviced not to skate it until 3-4pm.
Stargate is the usual meetup spot, and it's pretty central to anything in the city really.
Mølla is great on a friday night, and it seems like everyone who has ever touched a skateboard go to oslovelo for beers on fridays and/or saturdays if there isn't a rave somewhere.
reality is that there are tons of great bars in Oslo. some real high end and popular ones, and some really fucking shitty ones. you'll catch it by their looks, the ones that look like they are in the 80/90s based on their signage are usually brown holes (with cheap beer) and other ones are well, normal bars.
for food, Gaza has the best falafel wrap in oslo. burgers are a tie between Illegal, Munchies and Wunderburger. keep in mind that a burger with fries will run you like $20 and a falafel is $10.