Statoil has said the strike by 45 workers that started on midnight Monday at its Mongstad refinery near Bergen will have "major consequences" for its operations.
It warns there may be a near-halting of vessel traffic to and from Mongstad and potential rerouting of oil tanker deliveries from Norwegian offshore fields.
Deliveries to Mongstad will be reduced, with Norsk Hydro ASA's North Sea Troll B and C oil platforms able to continue by pipeline for at most two weeks, it said.
Statoil spokeswoman Ms Else Katrine Nesmoen said Mongstad's "storage facilities will probably be full within a week, or two, causing production at Troll B and C to cease if the strike continues beyond that".
But she said deliveries to the European continent will not be affected by the strike and that it will not create immediate problems for fuel and heating oil deliveries in Norway.
The strikers who work at the offsite facilities and the associated port were called out after the failure of pay talks between the Confederation of Vocational Unions and the Norwegian Oil Industry Association.