"The celebrating is over," Okubo told Kyodo news agency after the team's training in the Kagoshima Prefecture.
"It's the real deal from here on. I'm the only one on the squad who hasn't been picked throughout this whole time, and I've got a lot of catching up to do," said the 31-year-old, whose prolific form saw him emerge as the lead marksman in the J.League last term.
The former VfL Wolfsburg and Real Mallorca forward will vie with German-based Yuya Osako and Cerezo Osaka's Yoichiro Kakitani for the starting berth in Group C clashes against Ivory Coast, Colombia and Greece when the tournament begins on June 12.
"I'm confident I'll be playing," Okubo said.
"We're representing our country here. I've got to soak everything up as fast I can and make sure I'm on the same page as everyone else. I've got to be 100 percent."
Team mate Shinji Kagawa felt Okubo's unpredictability could make him dangerous for the opposition.
"Yoshito is the type of player who can do anything," said the Manchester United attacker.
"I like to think we combine well and we play at a similar pace. He's very good at pressing and taking the ball off opponents.
"We as a team have to figure out how to make the most of his qualities through the training sessions."
(Reporting by Amlan Chakraborty; editing by Patrick Johnston)
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