A third major national advertising agency has moved into the Washington market, although in this case its entry is into the field of public relations.

Ogilvy & Mather/Underwood, Jordan officially opened a joint Washington public relations venture yesterday, a somewhat delayed wedding reception for a merger consumated in February.

The association, with offices both here and in New York, was the first foray into the public relations business for Ogilvy & Mather, an international advertising agency. It also puts the agency in position to move into a new market in its old business -- advertising.

What Ogilvy & Mather is doing in Washington is hardly unique. J. Walter Thompson -- now operating largely through a subsidiary, Broulliard Communications -- and Needham Harper & Steers have been seeking Washington business more aggressively than before.

In part, the moves into this market by major national agencies reflect the advertising industry's enchantment with "issue advertising" or "advocacy advertising." Washington-born agencies also do a good deal of this type of advertising. The phenomenon, which goes by half a dozen different names, involves corporations and institutions coming to grips with policy questions that affect their image as well as their ability to peddle gasoline, wage packages, nuclear referenda or whatever it is they are selling.

To do this, clients are asking more from their advertising companies than just a catchy jingle or a pretty face in a television or newspaper advertisement: Now they want a publicly and politically compelling case presented in every arena.

"The world had changed. The world has become more complex, and we found ourselves being involved in more issues that were not just advertising issues," said Kenneth Roman, president of Ogilvy & Mather U.S. "When the energy crunch comes and Shell can't advertise gas, that's not just an advertising problem -- that's a communications problem."

Ogilvy & Mather devised a campaign that told customers how to make the most efficient use of their cars. By the end of this year, Shell will have handed out more than 1 billion Shell Answer Books with driving, auto purchase and maintenance tips.

"There are many ways you could go in Washington," said Roman. Among the possibilities for selling advertising are government accounts, local business and national business, including trade associations and unions, he said.

First, Ogilvy & Mather put its public relations business in operation here under Vice President L. Michael Dowling. Now Barbara Smith, a former account executive, has been named general manager for advertising in Washington to explore that market.