In the summer of 1995, he participated in Johnny's Pre-School, part of the NTV program Tensai, Takeshi no Genki ga Deru Terebi (天才・たけしの元気が出るテレビ!!). He passed the audition and joined Johnny & Associates at the age of 14. After being in Johnny's for only a short period of three months, he made his debut as the youngest member of the idol group V6. Unlike the rest of the members in the group, he did not have much experience as Johnny's Jr. The first time that he had been on a music program was only after the debut of V6. Their debut song was "Music For the People".
Okada can play the guitar, as well as the piano. His mother is a piano teacher. His voice is in the middle range, and hence his solos usually include slower ballads. Recently, he has been given longer solos in V6 songs, such as "Way of Life". He also reveals that during V6's 2008 concert Vibes, he was directing most of the lightning, staging, and costumes.
Beside being a V6 member, Okada is an accomplished actor with eleven movies, in eight of which he plays the lead, and sixteen dramas, in four of which he plays the lead. In a Shōnen Club Premium interview on May 7, 2009, he revealed that it was the Japanese drama Kisarazu Cat's Eye that caught the attention of viewers and directors, and made them aware of his acting ability. The success of the drama led to the making of two movie sequels, Kisarazu Cat's Eye: Nihon Series and Kisarazu Cat's Eye: World Series. His other notable dramas include Tiger & Dragon, a comedy drama about rakugo, a form of Japanese comedy acted only by one person telling that story. and SP (also known as Security Police), an action suspense drama about a team of security police bodyguards in charge of protecting important people in the government. With Shinichi Tsutsumi as his co-star, the drama drew in overall ratings of 15.35% despite its Saturday 11:00 p.m. (JST) time slot; its special episode broadcast in 2008 also received a viewership rating of 21.5%.
After several years of "drama hiatus", it has been announced in 2012 that Okada would play the main character in the 2014 NHK historical drama, Gunshi Kanbei (軍師官兵衛), set to air in January of that year. Kanbei, whose birth name was Yoshitaka Kuroda, was a man of ambition who served as the chief strategist under Toyotomi Hideyoshi in the Sengoku period.
He made a big leap in his movie career with Tokyo Tower in 2005. In the movie he portrayed a young college student torn between his love for a lady twenty years his senior and the views society has on these kinds of relationship. After that came Hana Yori mo Naho (花よりもなほ), known as Hana to Western movie audiences, directed by the renowned director Hirokazu Koreeda (Distance, After Life and Nobody Knows). In the movie, Okada portrayed Sōza, a samurai uninterested in killing his enemy and focused on what he could do today to be a better person instead. The movie earned him a nomination for the Blue Ribbon Awards, but he declined this nomination. Nevertheless, he still got awarded the Ishihara Yujiro New Actor Award at Nikkan Sports Film Award for his role in Hana. Recently, he has been involved in movies such as Kagehinata Ni Saku (陰日向に咲く, Flowers in the Shadow), portraying a young man in debt because of pachinko, and Otonari (おとなり), an unconventional love story between two neighbors who have never met each other and only follow the other person's life through the sounds they hear from the other side of the wall.
In 2012, Okada played the role Yasui Santetsu, an astronomer who invented the Jokyo calendar used for many decades, in the movie Tenchi Meisatsu (天地明察) directed by Yōjirō Takita. The director has previously won an Academy award for Best foreign-language film, Departures in 2009. Later that year, it was announced that Okada would appear in another film set for a spring 2013 release, Eien no zero (永遠の0), directed by Takashi Yamazaki. The film follows the journey of a kamikaze pilot named Kyuzo Miyabe, a man described as a coward yet volunteered to die for his country.
Okada is known for being reserved. He does not meet with many people within the agency. In an interview on Shōnen Club Premium, Okada said that when he was little, whenever his name was called up to read something in class, he would feel like vomiting and purposely avoided school on those days. Within V6, he is also the most quiet person. He enjoys carpentry and has carved wooden bears for children.
In 2010, Okada had reported in a press conference that he is certified to teach Jeet Kune Do and Kali in Japan, the latter was used by him as his main martial art when the SP series was filmed.