Mobil Corp., in a corporate reorganization that apparently presages new diversification, announced yesterday that its two top executives are relinquishing their posts as head of Mobil's oil subsidiary but will continue to run the parent holding company.

Rawleigh Warner Jr., 61, will remain chairman and chief executive officer of the parent company, and William P. Tavoulareas, 63, will continue as president. But as of Jan. 1, Allen Murray, 53, a career Mobil executive, will take over as president of Mobil Oil Co., the principal subsidiary.

Another Mobil veteran, Richard Tucker, 55, will become president of a new subsidiary to be called Mobil Diversified Businesses. This unit will make up Mobil's far-flung operations outside of oil--the struggling Montgomery Ward retail chain, Container Corp. of America, Mobil Chemical Co. and a variety of alternate-energy, minerals and coal subsidiaries.

In effect, a company official said, Warner and Tavaoulareas are installing trusted subordinates as heads of current operations, leaving themselves free to direct the parent company. The "real significance" of the announcment lies in the distinction it makes between Mobil Corp., the parent conglomerate, and the Mobil Oil Co., which formerly represented Mobil's only line of business, he said.

Mobil is the nation's second largest oil company, with worldwide sales of $68.6 billion in 1981.

The reorganization announcement gave no indication of what new directions the company might take. Mobil's previous attempts to expand and diversify have had a troubled history. The company failed in bids to acquire Conoco Inc. and Marathon Oil Co., and its acquisition of Montgomery Ward has been a drain on the company because the stores have lost money for several years.

Murray, who becomes president of Mobil Oil Co., was previously the subsidiary's executive vice president, as well as a director of the parent company. He has been with Mobil 30 years, starting as an accountant and working his way up to president of the U.S. marketing and refining division.

Tucker, who joined Mobil in 1961, has been serving as president of the Container Corp. of America division and of Mobil Chemical. In his new position, in which he takes on responsibility for the retail stores as well and reports to Tavoulareas, he apparently moves ahead of Stephen L. Pistner, the department store executive Mobil brought in last year to direct the so-far unsuccesssful effort to reverse the decline at Montgomery Ward.