First Interstate Bancorp of Los Angeles took advantage of the rushed-atmosphere surrounding the nation's second-largest bank failure to expand into Oklahoma on unusually favorable terms, banking experts said yesterday.

First Interstate had unusually strong bargaining power in negotiations with bank regulators because a federal law permitting the sale of the Oklahoma bank to an out-of-state institution expired at midnight Monday. On Monday evening, federal banking regulators arranged the sale of The First National Bank and Trust Co. of Oklahoma City, which had $1.6 billion of assets, including troubled real estate, energy and agriculture loans, to First Interstate.

"It is virtually a no-risk thing for us ," said Bob Campbell, senior spokesman for First Interstate, when asked about the deal. "The pressure was the expiration of the federal law. That helped us in our dealing."

"I think there were opportunities because of the pressure for regulators to do something," added Merrill Lynch analyst Frank Barkocy.

Under the terms of the deal, the Federal Deposit Insurance Corp., which insures bank deposits up to $100,000, is paying First Interstate $72 million to take over the troubled Oklahoma bank. The FDIC also has agreed to take over $418 million of troubled loans.

In the next four months, First Interstate has the opportunity to persuade the FDIC to assume additional troubled loans that it discovers. Following that period, First Interstate has a year to persuade the FDIC to assume an additional $250 million of loans that become troubled. First Interstate also will have the chance to earn fees by servicing these loans for the FDIC.

An FDIC spokesman, however, downplayed the significance of the expiration of the federal law. He said, "We were working on a deal for weeks. We didn't speed anything up, but we knew this was a very real deadline."

The sale of First National Bank and Trust Co. offers complete protection to the bank's depositors. First Interstate, the nation's ninth-largest bank holding company with $49.6 billion in assets, will assume about $1.5 billion in 33,000 deposit accounts and other liabilities.

First Interstate senior vice president Paul Minch said yesterday that even though the decline in oil prices and the value of real estate has depressed the economy in the Southwest, the bank is excited about the opportunity to enter Oklahoma City. He indicated that the favorable terms of the deal make this an excellent opportunity for First Interstate, which already has banking operations in 11 western states.

"Can you take a clean bank, in what is currently an economically depressed area, and do well with it? We feel the answer is 'yes' because we don't feel oil prices will stay depressed forever," Minch said, after explaining that the FDIC will assume the Oklahoma bank's problem loans.

"We are interested in the Southwest, which is a natural adjunct to our territory. It is a way to get into a banking market without paying a huge premium. Our strategy is to try to add markets without paying those huge premiums," Minch said. He added that First Interstate, which had net income of $313 million last year, would like to expand next into Texas, another state that has been hurt by the decline in oil prices.

The First National Bank and Trust Co.'s failure Monday night was second in size only to the 1974 collapse of Franklin National Bank, which had $3.6 billion in assets. First National's liquidity problems reached their peak on Monday when the Federal Reserve Bank of Kansas City, working closely with the Comptroller of the Currency and the FDIC, asked the bank to repay a $296 million loan.

When First National was unable to repay the Federal Reserve loan, the bank was closed and sold. It reopened for business on Tuesday under the control of First Interstate.