Oliver North's delayed criminal trial will begin next Tuesday, the judge ordered on Friday, and he gave North permission to use classified materials to try to prove he was acting under then-President Reagan's orders in some of his Iran-Contra dealings.

The ruling, by U.S. District Judge Gerhard A. Gesell, will allow government prosecutors to make a general "admission" to the jury, to the effect that the United States made deals with other countries to help the Nicaraguan Contras at a time when direct American aid was banned by Congress. North's lawyers will be allowed to present their own evidence and witnesses to add details.North's lawyers had objected to an earlier version of the prosecutor's "admission," saying that it would not allow them to try to show that Reagan participated "personally and directly" in offering inducements to the other countries.

They also want to offer evidence they say indicates that Reagan and other officials had established a policy of secrecy about the arrangement.

Most of the 12 criminal charges against North, a former National Security Council aide, involve hiding from Congress and the attorney general what the administration was doing to help the Contras.

Lawyers for the government refused to embrace defense contentions about possible concealment, Gesell said, prompting him to order that North may present evidence, if it exists, "to establish that any member of the National Security Council, the national security adviser, or the then-president or then-vice president" ordered North to cover up the arrangements.

North's lawyers have said they need to use secret documents to prove their case, something they would not have been allowed to do under earlier versions of the "admission."

Under his rulings, Gesell said, "the defendant will not stand in a worse position, and . . . his rights to a fair trial have not been prejudiced."

The trial start was set for 9:30 a.m. Tuesday in the capital's federal courthouse, on the same floor where most of the Watergate trials were held.

Prosecutor John Keker and chief defense lawyer Brendan Sullivan will deliver their opening statements on Tuesday after the jury is sworn. There had been six alternates but one, Tanya Mitchell, was excused Friday after she told the judge her co-workers had lobbied her about North, pro and con.

Gesell said in another written order that he accepts an agreement reached Wednesday by independent counsel Lawrence Walsh and Attorney General Dick Thornburgh.