Paul Williams is a legendary songwriter. He's won three Grammy Awards (plus an Oscar and a Golden Globe), worked with everyone from Barbra Streisand to Kermit the Frog, and, more recently, collaborated with electronic powerhouse Daft Punk. But, as he tells Oprah on "Super Soul Sunday," not all of his musical hits seemed destined for such success.

One of those songs was "We've Only Just Begun," made famous in 1970 by The Carpenters. But before the chart-topping brother-sister duo sang the sweet single, Williams had written the lyrics for a very different purpose: a commercial for a San Francisco bank.

The commercial for The Crocker Bank featured a montage of a young couple getting married, set to Williams' jingle.

We've only just begun to live

White lace and promises

A kiss for luck and we're on our way

Before the risin' sun, we fly

So many roads to choose

We'll start out walkin' and learn to run

And yes, we've just begun

At the end of the commercial, a tag line tied it all back to the bank: "You've got a long way to go. We'd like to help you get there."

"['We've Only Just Begun'] had all the romantic beginnings of a bank commercial," Williams tells Oprah in the above video. "Roger Nichols and I expanded it as a complete song and never in our wildest dreams imagined it would be a hit. And then an angel sang it. When Karen Carpenter sings your songs, you are blessed."

The same sentiment could be said for Barbra Streisand, who sang Williams' "Evergreen." The hit from "A Star Is Born" won the Oscar for Best Original Song in 1977. Being on stage with Streisand to accept the award back then was another particularly surreal experience for Williams.

"You're walking on that stage and you're looking out, [and] there's Kirk Douglas. There's Burt Lancaster. There's all these people who are these iconic greats," he recalls. "Someplace back inside all the charm and all the wit and all the 'aren't I just wonderful up here on this stage?' is a little piece of Paulie that goes, 'You don't belong here.'"

This sense of unworthiness is what Williams says drove him to drink. (He also says that one powerful act of love pulled him back from rock bottom.) Now sober, the songwriter believes that none of his professional successes measure up against the meaningful connection he's built with other alcoholics.

"The reward for that [professional achievement] is incomparable to the reward of talking to another alcoholic and knowing that we're both giving each other added breath, added life, in the exchange of recognizing a higher power working through both of us in this moment," Williams says. "I have never felt more useful in my life. I would not trade a day of this for all of that."

"Super Soul Sunday" airs Sundays on OWN at 11 a.m. ET, during which it also streams live on Oprah.com, Facebook.com/owntv and Facebook.com/supersoulsunday.