Oulu is easily accessible by plane, train, bus, or car. Oulu Airport is the second busiest airport in Finland and dozens of flights from Helsinki arrive there on a daily basis. There are connective flights to Oulu from all around the world via Helsinki.
It is easy to arrive to Oulu by train as well. There are several daily connections from Helsinki. Depending on the train connection you can travel from or via Tampere, Seinäjoki, Kuopio, Kokkola to Oulu as well. Connections are also from the north, e.g. Rovaniemi and Kemi. There are also night trains and car trains available when traveling to Oulu by train.
For travelers from closer areas, traveling by bus is ideal; you can get to Oulu from almost everywhere in Finland in a short period of time. The Oulu region and Pohjola Route is ideally experienced with a rental car while moving around in Oulu is convenient by bicycle or by local bus.