Nippon Mitsubishi Oil Corp. and Cosmo Oil Co. Ltd., Japan's largest and third-largest oil refiners respectively, will integrate their tanker operations in November as a streamlining step under a year-old alliance. The companies said they expect to cut their combined freight costs by at least $4.62 million a year through the move. Japanese oil companies are under tremendous pressure from larger foreign-owned rivals like Exxon Mobil Corp. to cut costs and boost efficiency to restore profitability, which has been severely eroded by their inability to fully pass along higher crude oil costs by raising prices. Under the agreement, Cosmo Oil will pay $258,500 to acquire a 35 percent stake in Nippon Mitsubishi's wholly owned tanker unit Nippon Ryoyu Tanker Co.