In real life, La Sûreté Nationale was directly replaced by the French National Police in 1966. Therefore it is somewhat anachronistic for Oscar to be using the term in an episode ostensibly set in 1977.
The Sûreté is one of two national police forces in France. Given that the Sûreté policed major urban centers, and the Gendarmarie Nationale took care of rural France, Pierre Lambert was speaking imprecisely when he said he wanted to meet Beaumont in a public part of Paris so that he "could call the gendarmes" if the meeting went sour. In fact, the gendarmes would not have jurisdictional authority in Paris.