Some 300 usually publicity shy employes of the Organization of American States stayed away from work yesterday and staged a protest vigil in front of their downtown headquarters as part of a long-simmering salary dispute.

The employes, one-third of the OAS work force here, held the day-long silent vigil at the Pan American Union, 17th Street and Constitution Avenue, NW, while the foreign ministers from the 27 OAS member nations met inside to discuss their demands.

The demands includes payment of $20.6 million in back salary dating to 1976, a 15 percent pay increase effective Jan. 1 and creation of a committee to establish a new salary system that would include automatic salary increases.

The employes allege that the OAS for the last four years has broken an agreement to keep their salaries comparable to those paid employes of the United Nations.

By late yesterday afternoon, the foreign ministers had taken no action on the issue, although the Venezuelan delegation apparently proposed that some of the back pay be paid. Observers said the issue would also be discussed today.

The 1,300 OAS employes here and elsewhere have received salary increases totaling only 12 percent since 1976, while U.N. workers have received 35 percent, said William Kuhn, vice president of the OAS Staff Committee, which represents the employes. OAS salaries range from $7,000 for custodians to $40,000 a year for senior staff officials, an OAS spokesman said. The average salary is $25,000. OAS employes, like those at the U.N., are exempt from U.S. personal income taxation.