Reese Witherspoon shared a sweet birthday shout-out to friend Oprah Winfrey this week ... a heartfelt message to the entertainment icon. “Happy birthday @Oprah!! ?,” Witherspoon, 43, tweeted.

Oprah Winfrey flew in yellow floral ... Even the cake that Winfrey brought out while singing “Happy Birthday” to King was decorated with yellow embellishments and candles.

The CBS Mornings co-host called the serenade from the vocal group the "biggest surprise" of her 68th birthday festivities ... and friends – including BFF Oprah Winfrey and her boyfriend Steadman ...