Following on from his 6-1 win over Anthony McGill in the last 32, O'Sullivan was never really tested in repelling the modest threat of Li, who spurned a few chances to make it a closer contest.
The five-times world champion contributed 59 in the opening frame before an 88 saw Hang level at 1-1.
But Englishman O'Sullivan ran in 90, 50, 40, 63 and 53 to ease into the last eight.
Welshman Mark Williams will renew old rivalries with O'Sullivan after his 6-4 win over Sam Baird.
Williams has not beaten O'Sullivan in competition since the 2002 Premier League.