It was her way to stand up to cancer.
"It's about living life and doing what I love," Serrano-Osorio said. "I may feel bad, but it's important for me to create memories for my children. With any disease you can sit in bed or live, and I chose to live."
Serrano-Osorio, a 32-year-old mother of two from Watsonville, is a lifelong A's fan who is now trying to inspire others by sharing the story of her fight against breast cancer. She was selected as the "Honorary Bat Girl" for the A's Mother's Day game against the Tigers.
Serrano-Osorio was picked through fan voting -- she says she had everyone in her hometown voting for her -- combined with input from players and celebrities, because of how she's fought her cancer.
In August 2010, Serrano was diagnosed with inflammatory breast cancer, a rare, aggressive form of the disease that affects younger women.
"It was a little earth-shattering," Serrano-Osorio said of her diagnosis. "I think when you hear cancer, my first thought was 'What do I need to do it beat it?"
She has gone through six rounds of chemotherapy and she entered a special clinical trial in April 2011. The cancer is now responding, but Serrano-Osorio knows she still has a long road.
"I will never be cancer-free," she said. "I will always be having some kind of treatment."
In the meantime, she is committed to enjoying her family and continuing to do the things she loves. She has her sights set on running another marathon.
"The Boston Marathon is a huge goal," Serrano-Osorio said. "I might not get there. Running 26 miles is a huge challenge. I can barely get through a half-marathon."
Serrano-Osorio also wants to continue seeing as many A's games as she can. She said she's been an A's fan her whole life, and treasures her memories of sitting in the bleachers and watching the Bash Brothers: Jose Canseco and Mark McGwire. She said she's been to Opening Day at the Coliseum eight years in a row, a streak she kept intact despite her cancer treatments.
"Just because you are diagnosed with breast cancer doesn't mean your life is over," she said. "You can fight and live."
Jeff Fletcheris a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.