The 67-year-old star - who is a spokesperson and director for WW, formerly known as Weight Watchers - shared a video of herself dumping the baked treat in the bin.
Posting the clip on Instagram, she said: "Time for a reset, clearing out the fridge.
"You know, we had 12 people over the house over the holidays - all stayed safe from COVID, thank God - and I gotta say, we ate every day like food had just been made.
"So, clearing out the fridge - no chance of one last piece of [Gayle King's] delicious Bananas Foster birthday cake."
Even Oprah over indulged during the festive season, but she makes sure to "get back in control" when the new year gets underway.
She explained: "That's what helps me close out the holidays, get back in control of how I wanna live.
"So let's do our 'day one' together. Not one day, not tomorrow, not next Monday, not next week. Let today be our day one, OK?"
She added: "And oh yeah - don't forget to hydrate!"
And she noted her post is a "sign" to people "waiting" for the right moment for their own reset.
She captioned the clip: "If you’ve been waiting for the right day, the right time, or some kind of sign…this is it. Today is the day. Day One starts now!"
While plenty of followers were inspired by the video, Dwayne 'The Rock Johnson' joked he was disappointed the cake was thrown away instead of making it to his house as his "cheat day" treat.
He replied: "You know I’m all about @ww and #OprahAndWW!! But I was a crazy kook yelling at my phone saying 'No no nooo I’ll eat that cake!! It’s my cheat day!! Send it my way!!!'
Happy Day 1!! (sic)"