Conservative Prime Minister Erna Solberg, who has been in power since 2013, secured a narrow victory with her coalition partner, the anti-immigration Progress Party, in September elections.
The new deal with the Liberal Party was announced by the three party chiefs after ten days of negotiations.
The distribution of ministerial portfolios between the three parties will take a little longer to agree, but an announcement is expected by Friday.
Traditional ally the Christian Democrats, who also has eight deputies, have so far resisted all offers to enter the current government.
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