The Haraldsheim hostel is ok. It is expensive (even for Norway) and a long way out of town. The only real choice is getting tram four to Sinsenkrysset but the way is not very well signposted. If you are coming from Oslo Gardermoen airport it is probably easier (and quicker and cheaper) to get the bus to Grefsen station and walk around the corner to the hostel. Being four kilometers out of town would not be a problem if there was no curfew at midnight -- so nightlife options are necessarily restricted if one doesn't want to be locked out of the accommodation.
It is a YHA hostel and has all of the usual institutional rules and seems to attract a lot of families. The breakfast is a huge Norwegian buffet and of cold meats and cheeses. It is almost worth paying the extra amount to stay here just for the breakfast. It has two hundred seventy beds, so it is large and tends to attract school or family groups.
The kitchen area is the main common area although it is not big enough should all two hundred seventy hostellers want to eat at the same time. In summer the outside terrace gets quite busy with people eating there. There are plenty of wooden chairs and tables overlooking a large sports field between the hostel and the tram stop. The dorm rooms have four beds and is kept very tidy but there is an almost total lack of air conditioning and the windows don’t open so it is almost impossible to sleep in the heat during the summer. On the other hand, the beds are comfortable and the showers are reliable.