Honduras' supreme court and congress had opposed the public vote, saying that Zelaya was trying to win support to allow presidents to serve more than a single term.
Zelaya has denied those claims, saying that the constitutional changes he sought were aimed at improving the lives of the poor.
Insulza said that many Honduran politicians consulted by the OAS delegation are opposed to the terms of the San Jose Accord, which was proposed by Oscar Arias, Costa Rica's president, a mediator in the dispute.
"The June 28 coup, Mr Micheletti's uncompromising power-grab, and the growing evidence of abuses of authority … cast a dark shadow over every aspect of preparations for the elections scheduled for November"
John Kerry, US senator and chaiman of Senate Foreign Relations Committee
"No one supported or was neutral on the return of Zelaya. They all opposed it. However, it is probable that some [would agree to Zelaya's return], with the proper guarantees that Mr Zelaya would not do certain things," Insulza said.
Under the terms of the accord, Zelaya would be reinstated as the Honduran president until elections are held in November and be granted amnesty for alleged offences.
In return, Zelaya would have to abandon plans to hold a referendum on changes to the constitution.
Insulza said that some Honduran officals had raised concerns about Zelaya being given amnesty for his alleged offences under the terms of the accord, but the OAS chief said that element of the proposal was necessary to promote political stability in Honduras.
"The return of President Zelaya without an amnesty would only send us into an inferno of crossed lawsuits, and justice tribunals coming and going," Insulza said.
Therefore, a "truth commission will be created to investigate exactly everything that happened, and that this amnesty applies only to political crimes," he said.
But on Tuesday, Roberto Micheletti, Honduras' interim president, suggested that only armed intervention could change the situation.
John Kerry, US senator and Democratic chairman of the Senate Foreign Relations Committee, said that Micheletti was choosing "crisis over resolution".
"The June 28 coup, Mr Micheletti's uncompromising power-grab, and the growing evidence of abuses of authority … cast a dark shadow over every aspect of preparations for the elections scheduled for November," he said in a statement.