The US$15.1-billion friendly takeover had broad implications in Canada and elsewhere due to Nexen's extensive holdings in Alberta's oilsands, Gulf of Mexico and the North Sea.
Nexen says the Committee on Foreign Investment in the United States has now given its approval, clearing the way to close the deal in the week of Feb. 25.
Shares in the Calgary company were up 60 cents to $27.53 in early trading on the Toronto Stock Exchange.
Canada approved the deal in December after extensive debate about ownership of major resources by foreign state-owned entities such as CNOOC.
The Harper government approved the Nexen takeover under the old rules but introduced new guidelines that will limit similar deals in the future.
China, Britain and the European Union have also given their go ahead for the Nexen takeover.
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