ＯＥＣＤの調査で北欧諸国が税率において世界の上位をキープした。

。

In an OECD survey, the Nordic countries maintained their top position in the world in tax rates.

Literal

OECD [possessive-の] survey [in-で] Nordic-countries [subject-が] tax-rate [in-において] world [possessive-の] top-ranks [object-を] kept.

において is a formal written equivalent of で meaning 'in' or 'regarding,' typical of news and academic writing. キープ is a loanword from English 'keep,' naturalized enough for news copy. 北欧諸国 uses the suffix 諸国 (various countries) common in geopolitical writing.