The songwriter, producer and artist died at his home in Westwood, Calif. surrounded by family and friends.
"Alan continued to write and perform until his last days," a statement from 1st Phase Records reads. "Alan was a generous man who gave his heart and soul to the music industry."
O'Day signed with Warner Bros. in 1971, later writing "Train of Thought" for Cher, "Rock and Roll Heaven" for the Righteous Brothers, and the 1974 No. 1 "Angie Baby" by Helen Reddy. Three years later he landed at the top of the Hot 100 with his own single, "Undercover Angel."
In the 1980s, O'Day teamed up with Janis Liebhart to co-write dozens of songs for the popular "Muppet Babies" cartoons.
Throughout his career, O'Day's songs were performed by artists ranging from Johnny Mathis, the 5th Dimension, Tom Jones, Dusty Springfield, Tony Orlando, Three Dog Night and Paul Anka, among others.
O'Day is survived by his wife, Yuka. Funeral services are pending.