The Calabasas professional staffing firm On Assignment Inc. plans to sell 6.65 million new shares of its common stock. It will also grant the offering’s underwriters a 30-day option to buy as many as 997,500 shares to cover over-allotments, the company said. Proceeds will be used for possible future acquisitions, working capital and capital expenses. On Assignment has about 26 million existing shares outstanding. The company, which provides nurses, lab professionals and other medical support staff, had been grappling with less than stellar earnings but largely rebounded in recent quarters. Company officials have said they intend to acquire other companies.