Oscar de la Renta   New York Magazine Pick   The Collection: Fresh cuts for uptown sophisticates. Sleek, slender shapes feature dramatic finishes like corseted backs and gathered bodices.   The Gown: Silk gown with jeweled buttons.   The Price: $5,500.   Where to Buy: Gowns range from $4,000 to $9,000 and can be purchased at Saks Fifth Avenue and Bergdorf Goodman.