Buzzfeed has a brief write-up on some of the previous appearances by Presidents & FLOTUSes at the Oscars:

buzzfeed.com

FDR gave a six-minute radio address from the White House during the 13th Academy Awards, in which he paid tribute to Hollywood’s stars, directors, and producers.

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President Ronald Reagan made a video cameo at the 53rd Academy Awards in 1980.

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First Lady Laura Bush appeared at the 74th Annual Academy Awards in 2002. She participated in a taped segment.

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Michelle’s appearance was nothing new, but people still freaked out, calling it “inappropriate.”