Okai passed the "Hello! Project Kids" audition in 2002. In 2004 Berryz Kobo was formed, with the intention of rotating the singers throughout the unit. Okai was not initially selected, and the idea was eventually dropped. Her sister, Asuna Okai, became a member of Hello! Pro Egg during the same year. The remaining children formed Cute in June 2005. The group did not make its official debut until late in 2006, with their first official single released in February 2007. In September 2007, Okai was added to a new unit called Athena & Robikerottsu with Risa Niigaki, Aika Mitsui, and Saki Nakajima. They sang the opening and ending theme songs for the anime Robby & Kerobby. The unit was effectively disbanded in March 2008, entering into an indefinite hiatus when the series ended.
On November 9, 2010, a solo video of Okai performing the dance to Cute's then-latest single "Dance de Bakōn" was released on the group's official YouTube channel. The video received more than 100,000 views in two days, and had received over 1.2 million views by May 2011. Following the video's success, more solo "dance-cover" videos of Okai were uploaded, including her cover of Aya Matsuura's song "Love Namidairo", which was released on iTunes on November 27 as Okai's first digital single. Her first solo live event, "Chisato Okai (Cute) Solo Live 2011 Vol.1: Kaisha de Odotte Mita!!" was held on January 25, 2011 at Pacific Heaven. A CD containing Okai's cover of Miki Fujimoto's "Romantic Ukare Mode", with "Love Namidairo" as the coupling track, was sold exclusively at the venue of her second solo live, but was also released digitally as part of Hello! Project's Hello Cover series.