Nearly eight months ago a lawyer then representing Oliver L. North said the Marine officer wanted "to step foward and take the spears in his own chest" in the Iran-contra affair. North testified yesterday that he planned to be the "scapegoat." But when his moment came to take the fall before the Iran-contra committees, he didn't. It turned out that North -- though willing to field a few spears -- had no intention of bearing the blame alone.

By the time he finished his first day on the witness stand, Lt. Col. North had in effect accused numerous Cabinet-level members of the Reagan administration of conniving to provide false information to the president, Congress and the American people. And he raised new problems for President Reagan with testimony that he had sent not one but five memorandums "up the line" seeking Reagan's approval for arms deals with Iran that included diversion of profits to aid the Nicaraguan contras.

"I thought I had received authority from the president," he said confidently. "I never carried out a single act, not one, . . . in which I did not have authority from my superiors."

As for the far-flung enterprises he undertook under the banner of "covert operations," North had no apologies. All were for a good cause, he insisted -- and all were approved by his superiors. Did he run a secret operation to raise money from private individuals to arm the contras? Yes, and he had no regrets. Did retired Air Force major general Richard V. Secord open secret Swiss bank accounts and handle millions of dollars off official ledgers as part of North's secret efforts? You bet.

North spoke disparagingly of "heroes" now coming forward who claim to have uncovered administration wrongdoing, apparently a reference to State Department legal adviser Abraham D. Sofaer and Assistant Attorney General Charles J. Cooper.

"I didn't make a lot of the decisions I'm accused of making," he said. Later, he referred bitterly to "a whole cadence of people" among top administration officials who "wittingly" knew of false cover stories being promulgated last fall after the Iran-contra scandal erupted, causing him to be fired from his National Security Council post. He added: "I didn't consider myself to be the lone wolf out here creating paper that nobody else knew about."

In a day of often fractious exchanges, emotional outbursts and duels between lawyers, the key moment came in late morning. North was being questioned sharply by John W. Nields Jr., chief counsel for the House select investigating committee, about his role in compiling false chronlogies to be used by Reagan administration officials last fall to explain arms shipments to Iran that had occurred in 1985.

"By putting out this false version of the facts," Nields asked him, "you were committing, were you not, the entire administration to telling a false story?"

North responded with obvious irritation and emotion.

"I'm not trying to pass the buck here," he said. "Okay. I did a lot of things, and I want to stand up and say that I'm proud of them. I don't want you to think, counsel, that I went about this all on my own. I realized there's a lot of folks around here that think there's a loose cannon on the gun deck of state at the NSC. That wasn't what I heard while I worked there. I've only heard it since I left. People used to walk up to me and tell me what a great job I was doing, and the fact is there were many many people, to include the former assistant to the president for national security affairs, the current national security adviser, the attorney general of the United States of America, the director of central intelligence, all of whom knew that {cover story} to be wrong."

The possibility that North might become something of a witness for the prosecution was not anticipated. Nor did it seem likely at the outset of yesterday's dramatic hearing.

When the twin mahogany doors of the Senate Caucus Room swung open at 9:07 a.m. to admit him, North strode purposefully and jauntily to the witness stand through the crush of photographers and standing-room-only spectators. Surprisingly, for someone who has cast such a long shadow, North appears slight and short. As he walked in, the old marble chamber was silent, aside from the whirr-and-click of cameras recording the moment. The noise suggested that the cicadas had returned to invade the Capitol.

North, smiling and wearing his Marine uniform with rows of combat ribbons, maintained an optimistic demeanor as lawyers sparred over the details of his testimony.

The first sign that North's day on the stand would be stormy came when the normally unemotional Nields reacted indignantly, even passionately, to North's defense of covert operations as essential to U.S. interests.

"It is a principal purpose of these hearings to replace secrecy and deception with disclosure and truth," Nields said. "And that's one of the reasons we have called you here, sir. And one question the American people would like to know the answer to is what did the president know about the diversion of the proceeds of Iranian arms sales to the contras. Can you tell us what you know about that, sir?"

North launched into his widely expected defense of the president. "I never personally discussed the use of residuals or profits from the sale of U.S. weapons to Iran for the purpose of supporting the Nicaraguan resistance with the president," he said, looking down at papers that he described as "notes," not a prepared text, on the table before him.

When North finished his obviously rehearsed statement, Nields jabbed like a middleweight: "You left something out, didn't you?"

Nields proceeded to establish that North "had a specific reason for believing" that the president had authorized the diversion plan outlined in memos that North had sent to his superior, then-Vice Adm. John M. Poindexter, for Reagan's approval.

Poindexter never instructed him not to send such documents for presidential approval, he testified. On the contrary, North said Poindexter "was specifically asking you to send memoranda up for the president's approval."

In some of the most heated exchanges since the congressional hearings began May 5, North's attorney, Brendan V. Sullivan Jr., tangled with Nields in attempts to switch the line of questioning away from that subject. Sullivan protested that North had answered questions "10 times" about his belief that the diversion had been authorized. But Nields persisted and elicited more damaging testimony.

In the end, North maintained that he had done what he had set out to do: Tell the truth -- "the good, the bad and the ugly . . . and to tell it all, pleasant and unpleasant."

Said North, "I am here to accept responsibility for that which I did." But, he continued, he is unwilling to "accept responsibility for that which I did not do."