Chavez had previously announced the government's intention to take a majority stake by May 1 in four heavy oil-upgrading projects run by British Petroleum PLC, Exxon Mobil Corp., Chevron Corp., ConocoPhillips Co., Total SA and Statoil ASA.
He said Monday that has decreed a law to proceed with the nationalizations that will see state oil company Petroleos de Venezuela SA, or PDVSA, taking at least a 60 percent stake in the projects.
"The privatization of oil in Venezuela has come to an end," he said on his weekday radio show, "Hello, President." "This marks the true nationalization of oil in Venezuela."
By May 1, "we will occupy these fields" and have the national flag flying on them, he said.
The law is expected to be published shortly in the government's official gazette, and the companies will have four months from then to negotiate terms and conditions with PDVSA to decide whether they will take part in new joint ventures as minority partners, Chavez said.
Chavez did not detail how the government will pay for its increased share in the projects in which the companies are estimated to have invested some $17 billion.
The government has compensated companies reasonably in recent weeks for nationalizations it has carried out in other sectors, but those agreements were for assets valued far less than the oil projects.
The Orinoco projects are the only oil-producing operations in the country remaining under private control, which Chavez called "disgraceful."
But he added that Venezuela doesn't "want the companies to go ... We just want them to be (minority) partners."
Also Monday, Chavez called President George W. Bush the "king of liars," predicting that the U.S. leader would not receive a warm reception on his upcoming Latin American tour.
"The South American people will give a welcome to the little gentleman from the North, the king of invaders, the king of liars," Chavez said referring to Bush's planned tour next month to Brazil, Uruguay, Colombia, Guatemala and Mexico.
"It is going to be more interesting that you think, the visit of the little gentleman," Chavez told his listeners on his weekday radio talk show, "Hello President."