Okada orders players to use oxygen tanks
Local media reported that Okada, whose team play two of their Group E games at more than 1,000 metres above sea level in South Africa in June, would ask his players to use the aparatus daily.
"After the squad is named I will get the players to use oxygen masks in small doses for an hour a day," Okada told the Nikkan Sports.
"They can use the tanks while watching TV."
Japan face World Cup-bound South Korea at home on May 24 before flying to Europe for warm-up matches against England in Austria on May 30 and Ivory Coast in Switzerland on June 4.
The Blue Samurai, who have never won a game at a World Cup tournament on foreign soil, have been drawn against Cameroon, Netherlands and Denmark in Group E.