OLDFIELD'S OLDIES: London designer Bruce Oldfield, who specializes in dressing British aristos, socialites and red-carpet celebrities, is opening his giant closet to the public. On Thursday, the designer — whose clients include Sienna Miller, Jemima Khan, Anjelica Huston and the late Princess Diana — plans to offer 300 one-off dresses from the past two decades during a one-day sale. The archive offer, however, is no harbinger of a company closure or impending sale, according to an Oldfield spokeswoman. "It's cleaning house more than anything else," she said. "It's better to have the pieces out in public than sitting in the warehouse. And it's a great time for vintage clothing." The sale will take place at London's Four Seasons Hotel on Park Lane.