Fat's In The Fire!

Diet divas Oprah & Marie Osmond in All-Out War.

A BITTER battle of the bulge is raging between Oprah Winfrey and Marie Osmond!

The diet divas are duking it out over who’s the biggest loser — Marie on Nutrisystem or Oprah on Weight Watchers!

Marie, 57, boasts she’s trimmed 50 pounds, and kept it off using the diet program she hawks.

Oprah, 62, owns a 10 percent chunk of rival Weight Watchers, and claims to have shed 40 pounds over 14 months.

And sources say Oprah ruffled toothy songbird Marie’s feathers when she crowed how she still drinks wine on Weight Watchers’ calorie-counting system.