Osmond, 60, donned a blonde wig — which was pulled back into a ponytail and featured large curled bangs — as well as a black-and-white striped jumpsuit.
She completed her look with bright red lipstick and matching-colored nails to imitate the “Hollaback Girl” singer.
Stefani, 50, responded by resharing the photo on her own Instagram page with the caption “@marieosmond literally blown away by this wow wow wow wow!!!!! Gx #amazing #ilovehalloween.”
Osmond then performed a “Sweet Escape” lip-sync on “The Talk” in competition with her fellow co-hosts.
“Oh my gosh, Marie, I just saw the ‘Sweet Escape’ Halloween performance,” Stefani said in her Instagram story. “I literally can’t believe you did that, I’m so honored.”
Osmond then shared a video of the performance to her own Instagram story, asking fans to vote for her in the friendly competition.
“Okay everybody, you need to vote for me,” Osmond said. “Why? Because I’m the newbie. Because I’m Gwen Stefani and because I’m 60 and I just rapped, come on!”
Osmond was joined by her co-hosts for the costume contest.