From the Wikipedia page 
Val-d'Oise (French pronunciation: [val dwaz]) is a French department, created in 1968 after the split of the Seine-et-Oise department and located in the Île-de-France region. In local slang, it is known as "quatre-vingt quinze" (i.e. "ninety-five") or "neuf cinq" (i.e. "nine five"). It gets its name from the Oise River, a major tributary of the Seine, which crosses the region after having started in Belgium and flowed through north-eastern France. Paris-Charles de Gaulle Airport, France's main international airport is partially located in Roissy-en-France, a commune of Val d'Oise.
In 1964, it was determined to divide up the departments of Seine and Seine-et-Oise. Val-d'Oise was one of the new departments so formed, and was created entirely from the previous department of Seine-et-Oise.
Val-d'Oise is a department in north central France and is part of the region of Île-de-France. To the south of the department lies the department of Hauts-de-Seine, to the southwest lies Yvelines, to the west lies Eure, to the north lies Oise, to the east lies Seine-et-Marne and to the southeast lies Seine-Saint-Denis.