"It's a very happy collection," said a relaxed Oscar de la Renta as he greeted guests filing into 583 Park Avenue, a venue that had been transformed by Stefan Beckman into a sort of minty-fresh aqua and white garden. Sure, the economy's barreling downhill, the Democratic nominees are locked in a grinding duel, and gas may hit $5 a gallon, but life does go on. And when it does, Oscar's lady who lounges, lunches, and loves to look fabulous will need something to wear. The designer covered her daytime needs with shantung skirtsuits, slim shift dresses, and high-waisted trousers, along with more playful options like a sparkling white sweater with an emerald gazar skirt. Bags and shoes came coordinated to the hilt, treading close to the point of distraction. But it was evening that was at the heart of this show. There was everything from a chocolate-box assortment of L.B.D.'s to sleek hammered-silk columns to a finale of massive, meringuelike ball gowns. An ocean blue stunner with a cascade of curlicue ruffles was pure de la Renta, while a white silk gown with a black bodice kept things simple. In these trying times, the good life doesn't always need to make a ruckus.