Fourth ranked Ruud raised his career record to 9-0 in Geneva, where he has won back-to-back titles over the past two years. The clay court tournament is the last warm-up before the French Open. Ruud was runner-up there last year.
Ruud committed a double fault on his first match point chance against the 49th-ranked American before scoring a second forehand winner as he advanced to the net.
“This is a great victory for me. JJ is an aggressive, tough fighter,” said Ruud, who will face him Nicholas Jarry in the quarter-finals on Thursday.
Jarry advanced as his opponent, the sixth seed Hellenic Tallon Railwaythey withdrew before the game.
O’Connell was leading 6-0, 3-0 against Bernabe Zapata Miralles when the seventh-seeded Spaniard withdrew from the match.