After months of partisan infighting among Washington's Hispanics over the appointment of an executive director of the Office of Latino Affairs, the mayor has chosen Aida Berio for the post, according to informed sources.

Berio, 50, is a former director of the city's Commission on Latino Community Development. She recently won a widely publicized discrimination suit against her employers at the U.S. Equal Employment Opportunity Commission, where she is now a national coordinator for field service operations.

Berio won her new job over the acting director of the Office of Latino Affairs, Carlos M. Rosario.

The office, which was made a cabinet-level slot in 1977,has thus far functioned largely as a referral agency for Latinos attempting to deal with the city's bureaucracy. Prior to 1977 it was a part of the Department of Human Resources.

Part of the controversy over the new appointment centered on the desire of many Latinos to make the office a more effective advocate for their needs.

The GS-15 executive director position was supposed to be filled on a strict merit basis. But backers of Rosario and, to a lesser extent, Berio, have been lobbying with Mayor Marion Barry for their favorites since this spring.

Rosario's supporters, in scores of letters and several meetings with Barry, pointed to his 25 years of service to Washington's growing Latino community. Many regard him as "the Godfather" of the city's Latino community, having virtually founded it.

Berio's backers, as well as others who opposed Rosario without supporting any specific candidate, alleged that Rosario tried to run the community as a political boss, showing conspicuous favoritism, ignoring some of its most pressing issues, and threatening opponents.

In recent weeks Berio also received substantial backing in the form of letters, phone calls and meetings with Barry by the D.C. Coalition for Women, of which she is a member.

Early last month, the Latino Commission gave Barry a list of five candidates ranked on the basis of their qualification. Berio was rated second, Rosario fifth.

Barry interviewed all five candidates and was expected to announce his decision more than two weeks ago. But for about the last 10 days, members of the Latino commission said, the mayor had clearly focused on Berio and Rosario without deciding between them. At a meeting with the commission last week, Barry suggested that both could be co-directors, but Berio flatly refused.

Formal announcement of the mayor's choice will take place today. Sources said Rosario has been offered a position as assistant director in DHR's office of aging. Berio would not comment and Rosario could not be reached yesterday. CAPTION: Picture, AIDA BERIO . . . D.C. women's coalition backed her