* Defense minister fired

* Oil, finance ministers unchanged (Adds background, details)

CARACAS, March 3 (Reuters) - Venezuela's President Hugo Chavez fired his defense minister on Tuesday in a minor cabinet reshuffle that left unchanged the key cabinet positions of oil minister, finance minister and vice-president.

Presidential spokesman Jesse Chacon announced the shuffle and said the housing ministry and the social protection ministry would be merged with larger ministries, in a bid to reduce spending as oil prices tumble in the OPEC nation.

Chacon said the changes were aimed at making the government more efficient. He did not say if government workers would be laid off as a result of the changes.

Chavez won a referendum vote in February that lifted term limits and will allow him to run for re-election as often as he pleases in South America's top oil exporter.

He typically reshuffles his cabinet after Venezuela's frequent elections and the nature of the changes can indicate his plans for the year.

This year's shuffle keeps in place men Chavez considers steady hands -- Oil Minister Rafael Ramirez, Finance Minister Ali Rodriguez and Vice President Ramon Carrizalez -- and does not bring back fiery ideologues who heralded a significant radicalization of Chavez's socialist revolution in 2007.

Last week Chavez reinstated leftist economist Jorge Giordani as planning minister. Giordani nominally heads the economic cabinet but is often overshadowed by the more powerful finance minister.