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The company said that de Castro is leaving to "seek new opportunities" after seven months at AOL.
De Castro is the latest top executive to leave AOL following Robert Pittman, who was squeezed out as AOL chief executive in July and Mayo Stuntz who quit his position as executive vice-president.
He will leave at the end of the month, after which time his organisation will report to AOL vice-chairman Ted Leonsis.
De Castro was central in shaping AOL's 8.0 software, which has been touted by chairman and chief executive officer Jon Miller as the company's most important product release yet.
"De Castro infused the whole place with new energy, and assembled a world-class team that has restored the sizzle of our brand by creating innovative, must-have new programming," Miller said.