I agree with Charles Eversole. I have been searching an answer for the same question and "Someone to Watch Over Me" is definitely not from Porgy and Bess. I found the following item on the internet, but it didn't "pan" out.
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GaryIndex
... AM I BLUE?, MIDI. SOMEONE TO WATCH OVER ME was written by George and Ira Gershwin
in 1926. It was one of the songs in the Broadway musical, OH, KAY! ...
www.geocities.com/Vienna/5702/midi.html - 9k - Cached - Similar pages

I found the list of lyrics for "Oh, Kay," and the song was not there. If anyone has the answer, please let me know. Thanks.