O'Sullivan was due to face either Michael White or Lu Haotian on Monday evening at the world ranking event in Chengdu. The winner of that match will now receive a bye to the last 16.
WPBSA chairman Jason Ferguson said: "We have been made aware that Ronnie has been unwell. The usual process for players pulling out of tournaments will now be applied.
"This is a huge disappointment for all of the fans in China who were hoping to see him play. However there is still an outstanding line-up of the world's best players at the International Championship and it will be a top class event."