A comprehensive network of Oslo hiking trails is accessible from the city metro system. All offer great access to the outdoors and are easy to reach from the city centre
The hiking trails around Oslo are similar to those in the rest of the country: clearly signposted, well maintained and often bookended with cosy cabins serving hot drinks and hearty food.
Trails in Norway are maintained by the excellent Norway Trekking Association (DNT). Their crowdsourced route planning sister site UT.no is also a great online resource full of hiking ideas, tips and maps.