An official of the Organization of American States who contradicted President Reagan's views on promises made by the Sandinista regime in Nicaragua to preserve democratic freedoms was placed on special leave yesterday.

Dan Cento, an OAS press officer, was placed on leave after refusing to resign, according to OAS sources. The OAS secretariat would not comment on his status last night.

"As of now, I'm not exactly sure what my position is," Cento said last night. "I've been put on special leave, and I expect to be hearing within the next 10 or 15 days."

In an interview Thursday with the Associated Press, Cento denied that the OAS had a "contract" with the Sandinistas for the implementation of democratic reforms after the overthrow of the Somoza regime in Nicaragua in July, 1979.

Earlier Thursday, Reagan said "the Sandinista government is in violation, literally, of a contract that they made with the Organization of American States." Cento responded that there was no such "contract" but only a resolution of a meeting of OAS foreign ministers held in Washington on June 23, 1979.

That resolution called for a solution to Nicaragua's problems "within the jurisdiction of the Nicaraguan people." It backed replacement of the Somoza regime by a democratic government that would guarantee human rights and free elections.

Aside from the broader political implications of Cento's reported remarks, OAS officials were said to be angered by the violation of their rule that only the organization's secretary-general, Alejandro Orfila, speak for the OAS.

The State Department, meanwhile, appeared to back away yesterday from Reagan's language. A spokesman said Reagan "was repeating the U.S. position that the Sandinista government has violated its pledge to the OAS as communicated to the OAS secretary-general in a telegram dated July 12, 1979.

"No vote has been taken on the Nicaraguan question in the OAS since 1979, but from our consultations within that organization, we know that other member states share our disillusionment with the Sandinistas on the fulfillment of these pledges."