OAS Secretary General Luis Almargo has called for sanctions against Venezuela.
At a summit of Caribbean countries in Havana, Mr Castro called the OAS "an instrument of imperialist domination".
Meanwhile, former Spanish Prime Minister Jose Luis Rodriguez Zapatero has met jailed Venezuelan opposition leader Leopoldo Lopez.
It was Leopoldo Lopez's first visit from anyone other than family members or lawyers since the 45-year-old was sentenced to 14 years in prison last year after being found guilty of inciting violence.
His supporters insist he is innocent and say he was jailed on trumped-up, politically-motivated charges.
The meeting at Ramo Verde military prison outside Caracas lasted about 90 minutes, according to  Adriana Lopez, the opposition leader's sister.
Mr Zapatero is trying to negotiate between the opposition and the government in Venezeula's worsening political crisis.
Cuba was expelled from the OAS in 1962 but following a recent thaw in relations with the US it was suggested that the island might return.
But Mr Castro appeared to rule out the possibility, offering "our most firm solidarity to our brothers the Venezuelan people, to the legitimate government of President Nicolas Maduro".
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Mr Maduro is locked in a dispute with the OAS over opposition demands in Venezuela for a recall referendum.
Mr Almargo said earlier this week that "the institutional crisis in Venezuela demands immediate changes in the actions of the executive branch".
He has called an emergency meeting of the OAS at which member states will decide whether to invoke the Inter-American Democratic Charter, which could lead to Venezuela's suspension from the regional group.
Cuban President Raul Castro says the country will not return to the Organization of American States (OAS) in a show of solidarity with Venezuela.