Richmond attorney Matthew Ott Jr., a criminal law specialist and former assistant state prosecutor, emerged yesterday as the leading contender to succeed U.S. Attorney William B. Cummings in Alexandria.

Ott, 43, an outsider to Democratic party circles, was named by the Justice Department yesterday as "under very serious consideration" for the appointment of chief U.S. prosecutor for the eastern district of Virginia.

According to a Justice Department spokesman, Ott heads a list of fewer than five candidates under consideration. Ott is scheduled to meet early next moth with Virginia's two U.S. senators prior to seeking congressisonal approval if he is selected.

"I would prefer not to comment," Ott said yesterday from his Richmond office. "I have no idea where I am on the list."

Ott said that he would meet with Sen. Harry F. Byrd, Jr. (I-Va.) and Sen. John Warner (R-Va.) next month.

The office is being vacated June 1 by Cummings, a Republican who was appointed by Presidents General R. Ford. Cummings' firsts deputy, Justin Williams, is expected to be appointed to the post on a temporary basis until President Carter's final choice is approved by Congress.

The job pays $47,500 a year.

Ott, a partner in the Richmond law firm of Ott, Morchower, Thompson and Mcmullan; is a former law clerk for U.S. District Court Judge Robert Merhige, a former assistant commonwealth's attorney in Henrico Country and served as a U.S. Magistrate from 1973 to 1975.