The Fairfax Symphony has hired Mark W. Ohnmacht, former general manager of the Nassau Symphony Orchestra in Garden City, N.Y., as executive director.

Ohnmacht succeeds Barbara B. Serage, managing director since 1972, who plans to leave the symphony in 1989. During the transition, Serage will be managing director for development and community relations.

Before going to Garden City, Ohnmacht was general manager of the Prince George's Philharmonic in Riverdale and, concurrently, was director of the Contemporary Music Forum in Washington from 1983 to 1985. He is a graduate of Catholic University.

In announcing Ohnmacht's appointment last week, symphony President John Cherin said the symphony started informally in 1956 with a group of "ladies around the table" and a budget of $10,000 to $15,000. It will launch its 31st season this fall with a budget of more than $600,000.

Symphony board member Ronald Geiger said the board approved the new position of executive director last spring and received 43 resumes from around the country.

Ohnmacht said he would like to see the orchestra increase touring outside Fairfax and to secure a permanent concert hall for the group. He said George Mason University hopes to open a performing arts facility with a 2,000-seat hall in the fall of 1989 and that it might be available for use by the orchestra that season.

James C. McKeever Jr., a board member and president of the Fairfax County Council of the Arts, said Fairfax County is also considering a performing arts center and is expected to appoint a task force to evaluate such a project next year.

He said any county concert hall would be three to five years away from construction.