The decision was made earlier this month by President Mahmoud Ahmadinejad, based on a recommendation by the board of directors of the fund, the Poul business daily reported, without citing any official sources.
State radio said the move was taken because the government wished to protect itself from the fragility of the U.S. economy and the weak dollar. The Central Bank of Iran, contacted by telephone, declined to comment.
The OSF, which forms part of Iran's foreign exchange reserves, is a contingency fund set aside by the government to cushion the economy against fluctuating international oil prices and help both the public and private sectors with their hard currency needs by extending loans.
The sizes of the OSF and the overall foreign exchange reserves are not regularly revealed to the public. Ahmadinejad said last December that the OSF was worth the equivalent of over $23 billion, and state television reported at the time that the reserves exceeded $80 billion.
The decision on calculating the OSF is the latest in a series of efforts by Iran, which is diplomatically hostile to the United States, to reduce the role of the dollar in its economy.
Iran has pushed for the Organization of the Petroleum Exporting Countries to switch from the dollar when calculating international oil prices, though it has so far received little support for the initiative.
The Iranian central bank has said it has been diversifying its reserves away from the dollar, but has not revealed details. (Writing by Hashem Kalantari; Editing by Andrew Torchia/Ruth Pitchford)
