The head of American Telephone & Telegraph Co.'s computer products division is leaving to rejoin the office equipment giant Ing. C. Olivetti SpA of Italy, AT&T announced Monday. Vittorio Cassoni, who had been head of Olivetti's North American operations before joining AT&T in 1986, will be nominated as managing director of the Italian company, AT&T said in a statement. Robert M. Kavner, AT&T's chief financial officer, was named to replace Cassoni as president of the company's Data Systems Group. The announcement came a week after Robert E. Allen succeeded James E. Olson as AT&T chairman. Olson, who had headed the telecommunications giant since September 1986, died of cancer. Cassoni, 45, joined AT&T after the American company and Olivetti signed a 10-year agreement under which Olivetti began to produce personal computers for both firms. Wall Street analysts said they did not know whether Cassoni's departure was being planned before Olson's death, although Allen said in a statement, ``When we brought Vittorio to AT&T in 1986, it was understood that he would eventually return to Olivetti.'' ``No one knows the answer to that,'' said John Bain, who tracks AT&T for the investment firm Shearson Lehman Hutton Inc. ``I'm tempted to say it must have been; they wouldn't undertake something at this high level at a whim.'' Olivetti was disappointed recently by AT&T's refusal to increase its 23 percent stake in the Italian company, but Bain said it was not known if there was a connection between that event and Cassoni's resignation. Analysts also said it was too early to tell what effect the change might have on the Data Systems Group, which is responsible for the development and marketing of AT&T's computer products. ``It is a loss,'' Thomas McKeever, an analyst at First Boston Corp. said of Cassoni's departure. However, McKeever added: ``A good chunk of his work is behind. They have laid the foundation and gotten a dedicated sales force.'' Bain, noting Cassoni's overhaul of AT&T's national marketing system, said, ``If he has gotten things turned about ... this could be viewed as a very positive step.'' But, he said, ``We don't know.'' Under Cassoni, the Data Systems Group agreed to develop a new version of AT&T's Unix computer with Sun Microsystems Inc. AT&T has also agreed to acquire a stake of up to 20 percent in Sun. The Data Systems Group is currently vying for a $4.5 billion contract to supply 20,000 computers to the Air Force and other defense agencies. Digital Equipment Corp. and Wang Laboratories Inc. dropped out of the bidding recently, claiming the deal favored AT&T. Bain said the Data Systems position could be a testing ground for Kavner. ``If he can be a general purpose executive rather than just an accountant, there could be higher things in the cards down the road,'' Bain said. Kavner, 44, joined AT&T in 1984 after 10 years as a partner at Coopers & Lybrand, the accounting and consulting firm.