Two former top executives of O.P.M. Leasing Services Inc. pleaded guilty last December to federal charges of defrauding 19 banks, insurance companies and other lending institutions of more than $200 million over a 10-year period.

The U.S. Attorney's Office in Manhattan announced today the unsealing of the guilty pleas of Mordecai Weissman, president of the company, and Myron S. Goodman, executive vice president.

When asked why the pleas had been sealed, the prosecutor in charge of the case stated that the investigation is continuing.

O.P.M. Leasing Services Inc. was a leading computer-leasing firm before filing for protection from creditors under Chapter XI of the bankruptcy laws on March 11, 1981.

The pleas were entered on Dec. 17, 1981, the same day that five former vice presidents of the company also pleaded guilty to various fraud charges.

Weissman, 34, of Lawrence, Long Island, N.Y., pleaded guilty to conspiracy and seven counts of wire fraud. Goodman, 35, who also lives in Lawrence pleaded guilty to 14 counts of wire fraud in addition to the conspiracy count. The conspiracy counts and each fraud count carry a maximum of five years in prison.

According to the criminal information, the defendants "forged, altered and otherwise falsified documents presented to the lending institutions to fraudulently induce them to extend credit or to purchase notes." According to the federal prosecutor in charge of the case, the defendants falsified leases that purportedly existed between O.P.M. and Rockwell International Corp., a major aerospace company, and persuaded the lenders to purchase 60 notes in 49 separate transactions.

The prosecutors said that the defrauded lending institutions were led to believe that the notes were secured by a security interest in the equipment purportedly leased to Rockwell. In most instances, the equipment purportedly leased to Rockwell did not exist, according to the government attorney.

Some of the lending institutions defrauded in the conspiracy were Chemical Bank, Chase Manhattan Bank, and Bankers Life and Casualty Co.