Can I order books from another library in Finland (e..g. Oulu Library) to be sent to Helsinki? If yes, what are the costs?
Yes you can if the material you need doesn't exist in libraries in the metropolitan area. The form for interlibrary loans you find here http://www.lib.hel.fi/page.asp?_item_id=3459 . Interlibrary loan inside Finland or from Nordic countries costs 4 €.