What was going on when I was born?

The nation's two largest steelmakers yesterday reported sizable first-quarter losses, reflecting the steel industry's continuing lag behind the nation's economic recovery. The United States Steel Corporation, the No.1 producer, lost $118 million in the first quarter, and the Bethlehem Steel Corporation reported a $175 million loss. The steelmaking segment at U.S. Steel, however, reported a $232 million operating loss, which the company said resulted from operating levels that were lower than last year. In the 1982 first quarter, U.S. Steel lost $24 million from steel operations.

The $225 million fraud by O.P.M. Leasing Services Inc. depended for its success on the ''ignorance, carelessness, poor judgment or selfinterest'' of many businessmen and professionals who dealt with the company, a bankruptcy trustee said in a report released yesterday. James P. Hassett, O.P.M.'s court-appointed trustee, said in the 600-page report that ''accountants, management consultants, lawyers, investment bankers, lessee representatives, bankers and other businessmen'' deserved ''substantial criticism'' for their dealings with O.P.M. His report said they all contributed, in some cases through dishonesty, to allowing ''one of the largest frauds in history'' to continue until years after it should have been exposed. Phony Computer Leases O.P.M., which grew to be one of the nation's largest computer leasing companies, obtained about $225 million from lending institutions during the decade before its collapse in early 1981 by using phony computer leases and other forged documents as collateral. In many cases, there were no computers. The company's two owners, Mordecai Weissman and Myron S. Goodman, and five employees have pleaded guilty to Federal crimes.

NEW YORK CITY supermarkets and specialty stores seem to be drowning in a sea of carbonated waters: naturally sparkling mineral water, naturally sparkling spring water, seltzer, sparkling seltzer, club soda, sparkling club soda, pure mineral water with effervescence, natural mineral water. The bewildering array of these products has engendered enormous confusion among consumers, most of whom are uncertain about the differences. Like many other products, especially those that have achieved a certain cachet, these bottled waters are often purchased for reasons that have nothing to do with their taste: How else to explain the popularity of a brand like Perrier, which even the former president of its American company could not differentiate from ordinary club soda in a blind tasting? And why would anyone spend $1.29 for a bottle indistinguishable from the contents of a 39-cent bottle of seltzer? Despite all the descriptive names given to these carbonated waters, there are only three categories: sparkling mineral water (carbonated water, often from an underground spring), seltzer (carbonated tap water) and club soda (carbonated tap water to which mineral salts are usually added). Everything else is simply a variation.

Sweden protested today against ''the gross violations of Swedish territorial integrity of which the Soviet Navy has been guilty'' and threatened to sink any submarine that enters its waters without permission. In an unusually strong series of actions for a neutral nation, the Swedish Government sent a stiff diplomatic note to Moscow, temporarily recalled its Ambassador there, Carl de Geer, and indicated that official visits between the two countries would be cut back sharply. Prime Minister Olof Palme said Soviet military activity off Sweden's coast should ''be roundly condemned by all.'' ''We are determined to be very firm but not hysterical,'' the Prime Minister said.

Former Vice President Walter F. Mondale charged yesterday that the Reagan Administration's arms policies were ''weakening America'' and proposed instead a series of specific steps, including a summit meeting with the Soviet Union, that he said would insure peace and national strength. In a frankly political speech to the nation's newspaper publishers, meeting in New York City, Mr. Mondale called the Administration's policies ''fundamentally flawed'' in their emphasis on building new, highly sophisticated weapons and in what he said was a rejection of real efforts to win accord on conventional and nuclear arms reductions. Freeze on Nuclear Tests Backed He backed a comprehensive and verifiable ban on developing nuclear weapons, including a freeze on nuclear weapons tests, and said the treaty to limit strategic arms negotiated by the Carter Administration in 1979 should be revised and resubmitted to the Senate. He endorsed a substantial buildup of conventional weapons as a way to reaffirm the policies of deterrence, but he opposed the development of a new experimental missile, the MX, and its deployment in existing silos.

At 4:30 yesterday afternoon, Bob West, a Treasury Department analyst, was standing amid a group of 20 people at the Merrill Lynch kiosk in Grand Central Station, catching the closing quotes before taking a commuter train to his home in White Plains. The Dow Jones industrial average, which most people use to gauge the stock market, was once again making history. It had just closed above 1,200 for the first time, specifically at 1,209.46, up 22.25 for the day. That is a rise of about 433 points, or 53 percent, since the start of the prolonged stock market rally last Aug. 12.

''If an unfriendly foreign power had attempted to impose on America the mediocre educational performance that exists today, we might well have viewed it as an act of war.'' - The National Commission on Excellence in Education. (A1:4.)

The Federal Trade Commission announced today that it was entering into an agreement with the General Motors Corporation setting up an arbitration program to resolve consumer complaints about engine and transmission problems. The program, which would not become final until after a 60-day comment period and might be modified, could involve several million customers who have bought General Motors cars produced since 1974. The agreement would settle a lawsuit brought by the trade commission against the corporation in 1980 that cited premature and excessive wear on the transmission, camshaft and fuel-injection systems of numerous G.M. models. It is apparently the first time case-by-case arbitration, rather than automatic direct relief, has been proposed in such a dispute. When the agency staff recommended the arbitration plan, F.T.C. attorneys said it reflected the Reagan Administration's desire to resolve regulatory matters quickly and noted that the usual approach would require extensive proof and might take years of court action.

The Monmouth County Prosecutor said today that the investigation surrounding the accused killer Richard F. Biegenwald would continue for two to three months and would take investigators to ''two additional sites.'' The Prosecutor, Alexander D. Lehrer, refused to say what investigators would be looking for. They have discovered the bodies of five murder victims linked to Mr. Biegenwald on Staten Island and at three sites in Monmouth County since last January.

WHATEVER the ultimate significance of the current season, it is clear that so far the collections have not been much fun to watch. This week the big-name designers took to the runways, scheduling their showings back to back. There was hardly time for the audience to get from one place to another. While there have been attempts at superproductions in surprise places, they have usually been failures. Perhaps too much energy went into the staging, too little into the clothes. At any rate, the most successful presentations were those staged unpretentiously in the company's showrooms.

The Signal Companies said yesterday that it was reorganizing itself as a high-technology and engineering company and would dispose of its money-losing Mack Trucks division. In order to divest Mack Trucks, which accounted for a quarter of Signal's sales last year but lost $27.5 million, Signal said that it had begun discussions with Regie Nationale des Usines Renault, the French auto maker that already owns 20 percent of Mack. Signal said that it expected Renault to acquire enough of Mack to give it a controlling interest but not all of the company. The balance of Mack will then be disposed of through a public offering or a distribution to Signal's shareholders.

Mayor Koch said yesterday that the city's next Schools Chancellor should be one of four finalists who lost out to Deputy Mayor Robert F. Wagner Jr. almost three weeks ago. The Board of Education picked Mr. Wagner for the job, but he was rejected by the State Education Commissioner last week. ''I think they hunted and they came out with 70 applicants, 18 indepth interviews and 5 they found very qualified,'' the Mayor said in an interview and later in a statement released by his press office. Mr. Koch, who had wanted Mr. Wagner to be Chancellor, declined to say which of the four runners-up he preferred for the $85,000-a-year job. The Mayor spoke out as the board, unsure about how to proceed and concerned about a lawsuit questioning its practice of convening in closed sessions, canceled a meeting planned for yesterday and rescheduled it for today.