A Silver Spring dentist who witnesses said freely dispensed narcotics to some of his friends and associates and conducted sex orgies at his office and home was convicted yesterday of 70 drug offenses following a five-week trial said to be among the longest ever held in Montgomery County.

Dr. Dennis S. Oddo, 29, theoretically could be sentenced to almost a thousand years in prison for the offenses, which included illegal distribution, dispensing and possession of drugs, principally the narcotic painkiller Demerol.

A Montgomery County Circuit Court jury returned its verdict -- including findings of innocent of 11 drug charges and two charges of maintaining a disorderly house -- after three days of deliberation.

Judge Joseph Mathias scheduled sentencing for Sept. 22 and released Oddo under $150,000 bond.

Peter Davis, one of Oddo's attorneys, said an appeal is planned.

County police arrested Oddo in his office last August following a six-month investigation spurred by a Kensington pharmacist's concern about his prescribing practices. The pharmacist told police he believed that Oddo was "obtaining excessive amounts of Demerol." Oddo's prescriptions, according to court records, were made out to both patients and acquaintances.

Several prosecution witnesses were Oddo's patients, who testified they had no knowledge of Demerol prescriptions made out in their names.

Ellen Hicks, Oddo's former assistant, who was given immunity from prosecution in exchange for her testimony, told the jury that at first the dentist supplied drugs to his staff and friends only occasionally. Over a few months, she testified, use of Demerol in the office and at parties became more frequent as did her trips to purchase the narcotic with prescriptions made out to patients and staff members.

Robert Newmark, a former student at the Georgetown University dental school who was acquainted with Oddo, testified that Oddo, who is married and the father of a child held drug and sex orgies and took Polaroid pictures at them.

Newmark also testified that Oddo offered him Demerol three or four times during his visits to the dentist's office even though the drug was not required for medical treatment.

During at least one of these visits, Newmark said, he, Oddo, Hicks and another assistant injected Demerol together in the dentist's office. He said he also recalled a party given by Oddo at which the dentist and four others injected Demerol and took Quaaludes a tranquilizer.

Oddo's attorneys argued that the prosecution witnesses were unreliable because they had been given immunity from prosecution in return for their testimony at the trial.

The defense contended that since a large number of Oddo's patients were handicapped, he or his staff would often pick up prescriptions themselves. Oddo employed an intravenous sedation technique in which a patient is injected with a combination of drugs that have an amnesiac effect after dental work is completed.

One of Oddo's handicapped patients testified for the defense she had received the sedation and felt no pain and had no recollection of the root canal work done on her.

As the jury deliberated yesterday morning, Oddo, who did not testify at the trial, told a reporter he was "not at all prepared for a guilty verdict."

When the jury returned shortly after noon he sat tense but impassively as the jury foreman delivered the verdicts on each of 83 counts.

The county grand jury originaly indicted Oddo on 88 charges -- believed to be the largest indictment in county history. Judge Mathias dismissed five of these charges before the case went to the jury.