The board of directors at Moss Airport Rygge, located some 60 kilometres south of Oslo, said last week that Ryanair's departure would doom the airport to closure.
"The Oslo base will close in October," Ryanair's chief commercial officer David O'Brien told reporters as an 80 Norwegian kroner ($10, €9) passenger tax came into force.
O'Brien said Norwegians should blame the government for the "sad, sad news" and emphasized that the tax was "the only reason" it would pull out of its Rygge base.
In a press release, the company went on to blast the government over the new tax.
"The illogical decision of the Norwegian government to introduce a flat rate environmentally unfriendly tax unfairly penalises passengers on efficient, green, airlines such as Ryanair in favour of passengers on high fare, half empty, gas guzzling airlines, and destroys the cost competitiveness of privately owned Oslo Rygge Airport in favour of the state owned Avinor monopoly," O'Brien said.
The decision means 16 routes will be cancelled and the reduction of Ryanair's traffic in Norway by 50 percent, the company said in a statement.
Airport managers had warned they would have to shut down Rygge should Ryanair leave, because the low cost operator runs the bulk of flights in and out.
Should that happen, 1,000 people would be left jobless, they said.
"There is still an opportunity. If the tax is scrapped ... in fact we won't close the base and we will reopen it depending on when these decisions are made," O'Brien said.
The Norwegian government has rejected Ryanair's demands.
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"I won't let my choices be dictated by Ryanair," Prime Minister Erna Solberg has previously said.
"The tax is implemented from today," she reiterated on Wednesday.
Ryanair's remaining operations will be moved to another privately-runairport named Torp Sandefjord some 90 kilometres southwest of Oslo, O'Brien said.
He meanwhile announced the opening of a new route between Gardermoen, Oslo's international airport, and London Stansted.