The Norwegian Petroleum Directorate said 1.66 million barrels of oil were produced in the country last month on average. That's 13 percent lower than May 2013, but 5 percent above what it had expected for the month.
"So far this year the oil production is about 2 percent above the NPD's prognosis," the agency said in a statement.
NPD blamed the lower output for May on reduced production from the Brage, Grane, Heidrun, Oseberg and Veslefrikk fields, which are closed for routine maintenance work.
NPD said it expected its estimate of offshore reserves could grow by 7.5 billion barrels of oil during the next decade.
Norway is the largest oil producer and the third-largest natural gas producer in Europe. Most of its oil went to the United Kingdom and the Netherlands last year. It supplied members of the European Union with about 21 percent of their gas needs last year, just behind Russia.