The NOK516m (£47.5m) contract covers upgrades to the Ekeberg and Svartdal tunnels on the E6/E18.
The project consists mainly of electrical work, fire protection and an improved cleaning solution for water from the tunnels. In addition, new service buildings will be built, evacuation routes will be upgraded, a new layer of asphalt will be added and all signs will be replaced.
The work will largely take place at night time in order to minimise the impact on traffic.
Renovation will begin this summer for completion before summer 2019.