Who sang the most Oscar®-winning songs in their original films?

Bing Crosby

4 Best Song winners:
"Sweet Leilani" from Waikiki Wedding (1937)
"White Christmas" from Holiday Inn (1942)
"Swinging on a Star" from Going My Way (1944)
and "In the Cool, Cool, Cool of the Evening" from Here Comes the Groom (1951)

Crosby also performed the following Best Song nominees:"Love in Bloom" from She Loves Me Not (1934)
"Pennies from Heaven" from Pennies From Heaven (1936, uncredited)
"Only Forever" from Rhythm on the River (1940)
"Ac-Cen-Tchu-Ate the Positive" from Here Come the Waves (1945)
"You Keep Coming Back Like a Song" from Blue Skies (1946)
"Count Your Blessings Instead of Sheep" from White Christmas (1954)
"True Love" from High Society (1956, uncredited).