Executive orders originated under George Washington, and their use stems from interpretations of Article II of the Constitution — which created the executive branch — and from presidential precedent.
Obama has not issued 900 executive orders. He has signed slightly fewer orders than President George W. Bush during this point in his first term, according to the University of California, Santa Barbara, which tracks executive orders. Obama has issued 139 executive orders as of Sept. 25. (The U.C. website listed 138 orders on Sept. 25, the same day Obama signed order 139). Bush issued 160 executive orders through Sept. 20, 2004, a comparable amount of time.
The viral email that claims Obama has signed 900 executive orders lists 13 orders as evidence, all of which previous presidents signed in the 1960s and 1970s.
Presidents number their executive orders consecutively. The first executive order that President Obama signed was EO-13489, which dealt with presidential records. Obama’s predecessors signed any executive order with a number lower than 13489.