Born in Tokyo, Japan, Oka's parents divorced when he was one month old. His mother moved to Los Angeles, California, United States when he was six. He was raised in a single parent family, and has never met his father. In 1987, a 12-year-old Oka was featured on the cover of Time Magazine titled "Those Asian-American Whiz Kids". Though he was not featured in the article itself, he was acquainted with the photographer who conducted the shoot. He has an IQ of 180. Ten years later, he graduated from Brown University in 1997 with a Bachelor of Science degree in computer science and mathematics.

Oka landed his first job after graduation at Industrial Light & Magic, George Lucas's motion picture visual special effects company, with the hope of one day earning an Oscar for technical work on a motion picture. His first major project was co-developing a computer program to generate water effects, which was first used for A Perfect Storm but also used in later films such as Pirates of the Caribbean: Dead Man's Chest. He later created programs for computational fluid dynamics and surface-tracking, which also became used in multiple projects. He has also worked on the Star Wars prequel trilogy

Oka first tried his hand at acting in 2000. He earned a Screen Actors Guild card by appearing in industry films, then moved to Los Angeles. ILM stipulated in his contract that he could work at its Los Angeles branch, but would have to return to San Francisco if he did not get cast for a recurring role that season. Oka did get cast for a pilot. Although the show was not picked up, it did satisfy the contract's requirements and he was allowed to stay in Los Angeles. He decided to continue pursuing acting. He landed several guest spots and then a recurring role as Franklyn on the NBC comedy Scrubs. He was also featured in a PS2's Shinobi commercial, with the catch phrase 'Shinobi's Back'. Oka guest starred in the Yes, Dear episode Dances with Couch, aired April 8, 2002.

In 2006, Oka was cast for the role of Hiro Nakamura in Heroes. Oka translates his dialogue from the show's scripts from English to Japanese; English subtitles accompany his Japanese dialogue on the show. Many of his co-stars portraying Japanese characters speak with thick American accents. Oka's accent is only slight, though noticeable to any native Japanese speaker. Additionally, Oka's real voice is closer to that of "Future Hiro" than the higher-pitched voice he uses as "Present Hiro" as heard during his appearances on television shows like Scrubs.

His portrayal of the role earned him a Golden Globe Award nomination that year in the "Best Supporting Actor on a TV Series" category. In addition to his work on the show, he continues to work at ILM up to three days a week as a Research and Development Technical Director, writing programs that create special effects. He will play the role of Bruce in the big screen version of Get Smart.

He recently signed on to play a Realtor who is trying to sell to Sean William Scott's character in Steve Conrad's Quebec. The film was originally slated to hit theatres in May 2007 but new scenes and characters have been added.

He was named the "Coolest Geek" at the Spike TV Guys' Choice Awards on June 13, 2007. In 2007 he presented an award with costar Hayden Panettiere on the 20th annual Nickelodeon Kids' Choice Awards.