The Washington Opera has turned to one of the orphans of the Mobil-Exxon merger for new administrative talent. Walter Arnheim, the former treasurer of Mobil Corp., has been appointed the company's chief operating officer, effective Jan. 1.

Arnheim will effectively replace Patricia Mossel, the company's executive director since 1995. Mossel will continue with the opera as a consultant. Arnheim's position as chief operating officer is a new one; he will be nominated to become executive director on July 1. The COO position may be eliminated at that point, according to an opera spokesman.

Placido Domingo, whose artistic leadership has spearheaded a huge expansion in the company's budget and fund-raising since he joined the opera in 1996, will continue as artistic director.

Arnheim has been a member of the opera's board of directors since 1996, when Mobil became the chief corporate sponsor of the company. He was Mobil's representative to the board.

Of his departure from Mobil, Arnheim says: "It has allowed me to pursue other opportunities, and I want to take advantage of that. When you get paid, you get paid in money and in intangibles, and the intangibles--the rewards of being associated with this company--are well worth it."

Arnheim, 55, was born in Pittsburgh and has lived in New York, London and Paris, the three opera capitals of the world. He says he is a longtime admirer of Domingo, and his personal taste in opera runs to the heavier German repertoire, especially Wagner and Strauss.

He says managing the rapid expansion is one of the key issues facing the company.

"When you see something like that, it's hard to provide the infrastructure, to build on the strength of the organization that is already there. I don't believe the Washington Opera is anywhere near finished growing. I think the leadership of Domingo, his artistic vision, is challenging the organization. I want to enable the Washington Opera to meet that challenge."

Arnheim's appointment brings serious expertise from the for-profit business world to the nonprofit opera world, part of a growing trend among nonprofits (and particularly nonprofit arts groups). Arnheim cites the appointment of Lawrence Small as the new head of the Smithsonian Institution as another example of tapping corporate talent. Small was the number two executive at Fannie Mae, the Washington-based financial giant, before accepting the Smithsonian job. Mossel cites the opera's growth, from a budget of $11.7 million in 1995 to more than $26 million today, as one of her major accomplishments. She was also a strong supporter of plans to convert the Woodward & Lothrop building to a new home for the company, a project that fell apart last year when the expected price tag for renovating the downtown building exceeded $200 million.

"That was a major disappointment," she says. Mossel says she is "keeping my options open" and that she plans to spend time on other interests, including her family, painting and finishing a book.

CAPTION: Patricia Mossel will be replaced as the opera's executive director.