For months, the public and much of the news media have come to expect that Oliver L. North's testimony to the congressional investigating committees would provide a climactic moment in the Iran-contra affair. But senior members of the committees and their staff have different expectations.

North, say committee sources, is not being called just to tell what he knows about some "secret government," or to provide explanations for his free security fence, or even just to answer whether the president knew of a diversion of funds to the contras. Of greater importance to the committees is the help North can provide in establishing the roles of those ultimately responsible for his questionable activities: the president and his senior Cabinet members.

Indeed, some committee sources express concern that the publicity devoted this week to North and next week to his former boss, ex-national security adviser John M. Poindexter, may distract attention from more fundamental questions of responsibility for the Iran-contra affair.

The committees now plan five more weeks of testimony -- three weeks beyond North and Poindexter. After seven weeks of preliminaries, these hearings are intended to take the investigation in a new direction: upward. In this final phase, the committees will hear from the senior-most officials involved: Attorney General Edwin Meese III, Secretary of State George P. Shultz, Secretary of Defense Caspar W. Weinberger, former White House chief of staff Donald T. Regan and Deputy Director of Central Intelligence Robert M. Gates.

In seven weeks of public sessions, testimony from lower-level officials has documented abuses of power, deception of Congress, unauthorized or illegal activities, and attempted cover-up. Initially, many of the administration's allies hoped to isolate the blame for all questionable actions on North, other low-ranking figures and perhaps the late director of central intelligence, William J. Casey. But as the testimony has unfolded, the involvement of numerous other officials in many aspects of the affair -- including President Reagan himself and all his principal Cabinet deputies -- has become clearer.

The investigating committees hope North will be able to provide new information on his own dealings with superiors. Documents and testimony produced by the panels so far have shown that North was almost obsessive in keeping his superiors informed about his activities, and that he generally sought authority for what he did.

Committee sources believe, for example, that someone at the White House approved the highly controversial "nine points" that were negotiated in Frankfurt last October and led to a final shipment of U.S. arms to Iran and the release of a U.S. hostage just before the congressional election. Among the points was the suggestion that the United States would defend Iran in a war against the Soviet Union. North signed off on the initiative, but committee sources say they have reason to believe he cleared it with higher authorities, as he regularly did.

In contrast to the picture painted by the Tower review board of a "disengaged" Reagan, the congressional hearings have depicted a president who directed or knew about many of the questionable activities taking place underneath him.

In March 1985, when U.S. assistance to the Nicaraguan contras was prohibited by law, Reagan was told by King Fahd of Saudi Arabia in a private meeting of the two leaders that a multimillion-dollar contribution to the contras, already solicited by U.S. officials, would be forthcoming.

The president, however, did not inform his own secretary of state of this when Shultz debriefed him immediately after the meeting. Congressional sources said this episode suggested that Reagan may have understood there were some things he was not supposed to know, perhaps because they were illegal.

Reagan also knew that in November 1985, Israel was going to transfer U.S.-made Hawk missiles to Iran. Government attorneys later determined that the transfer violated the Arms Export Control Act. In addition, the covert use of Central Intelligence Agency personnel and a CIA proprietary airline to assist the Israelis required a presidential authorization in advance, but no such approval was drawn up until after the arms transfer. An after-the-fact authorization was drafted for Reagan's signature and sent to the White House by Casey.

The original copy of that key document is missing, and North, who told CIA officials that it had been signed, may be able to clear up the mystery of what happened to it.

After the story of U.S. arms sales to Iran first broke, sources say, the president set the tone for the administration's response to the disclosures when he told reporters Nov. 6 that there was "no foundation" for the reports. According to former national security adviser Robert C. McFarlane's testimony to the Tower board, the White House staff had agreed earlier only to make no comment on the reports while efforts continued to free U.S. hostages in Lebanon.

Soon afterward senior White House officials joined in efforts to conceal the president's active role in critical aspects of the affair, according to earlier testimony to the committees.

According to testimony from Assistant Attorney General Charles J. Cooper, the NSC staff was in the process of altering a generally accurate Nov. 17 written chronology of the Iran initiatives at the same time it was preparing Reagan for his Nov. 19 news conference. The main changes in the White House chronology eliminated the president's role in giving advance approval to the apparently illegal transfers of U.S.-made weapons from Israel to Iran in 1985 -- transfers the president did not describe in his news conference.

North supervised the preparation of the original chronology and the subsequent alterations of it.

The assertions by Cabinet members that they simply were not told, or were misinformed, about activities in the Iran-contra affair resembled some of the initial statements from the White House. But testimony has suggested that Cabinet members also knew more than they said initially, and may have been caught up in the cover-up.

Shultz has repeatedly testified that he protested the original transfer of U.S. arms from Israel to Iran, and also disapproved of the subsequent direct sales of American arms to Tehran. He has said that although McFarlane briefed him in advance of the controversial November 1985 transfer, it was too late to stop the transaction. But documents released two weeks ago show that the U.S. diplomat in charge in Lisbon had become involved in helping to arrange the transfer and was told his help had received top-level State Department clearance.

Shultz has insisted that, along with his chief deputy on contra matters, Assistant Secretary of State Elliott Abrams, he knew nothing about the extent of support North and the White House were providing to Nicaraguan rebels. He did not ask about the source of funds that he had been told had been raised to help the contras, and he did not directly ask the president what North was doing in Central America, even though he told Abrams that he believed North was a "loose cannon."

Shultz did not pursue his opposition to the arms transfers to Iran with the president even after learning that the shipments were continuing. Furthermore, he ignored the administration's own covert policy and lobbied with other nations for a tough, no-compromise U.S. policy toward terrorists and hostage-taking.

Days after the U.S. role was discovered last November, Shultz, who was abroad, suggested in a cable to the White House that the administration acknowledge only one arms shipment -- rather than the seven that had taken place -- and emphasize that it was undertaken on humanitarian grounds.

This advice was turned down, and North went to Geneva on Nov. 8 to make one last unsuccessful effort with the Iranian intermediaries to get the remaining American hostages freed.

It was after this effort collapsed that the president and his top advisers met on Nov. 10, with Shultz present, to map out what some congressional investigators now say they believe was the beginning of the cover-up.