Washington Mutual Inc.'s $6.3-billion purchase of H.F. Ahmanson & Co., which combines the nation's two biggest savings and loans, won approval from the Office of Thrift Supervision. The pact creates the seventh-largest banking organization in the U.S. with assets of $150 billion with 2,000 offices. Shareholders of both companies approved the combination Aug. 28. Ahmanson, based in Irwindale, is parent of Home Savings of America. Under the agreement, Home will merge into Washington Mutual Bank and Ahmanson will merge into Washington Mutual Inc., a Seattle-based holding company. Washington Mutual Inc. has assets of nearly $97 billion, and Ahmanson has assets of about $56 billion. The two S&Ls; have more than 21,000 employees. It will be the second-largest deposit holder in the western U.S. behind BankAmerica Corp. The companies expect to complete the transaction around Oct. 1. Washington Mutual shares closed up 69 cents to $35.13 on Nasdaq. Ahmanson rose $1.69 to $58.31 on the NYSE.