PARIS, Sept 15 - The Paris-based OECD thinktank said on Monday it estimated that gross domestic product in the OECD area increased by 0.5 percent in the second quarter, up from 0.3 percent growth in the first quarter. Releasing its preliminary estimates for the second quarter of 2003, the Organisation for Economic Cooperation and Development put U.S. growth at 0.8 percent, up sharply from 0.4 percent growth in the first. Quarterly growth in the euro area turned negative -- down 0.1 percent in the second quarter after a flat first quarter. Japanese GDP surged by 1.0 percent, following robust first-quarter growth of 0.6 percent, giving Japan the strongest annual growth rate of the OECD block. The OECD area covers 30 countries.//