by Adam Lucius, Omnisport
The reigning KFC Big Bash League and Champions League victors were restricted to 9-113 from their 20 overs before following up with a dismal bowling and fielding effort, allowing previously winless Perth the easiest of victories.
The out-of-character display stopped the Sixers' 12-match winning streak dead in its tracks.
O'Keefe was one of the few players to make a meaningful contribution, top scoring with 29 to help drag the hosts from a parlous 6-28.
He also claimed a wicket but took little joy from his own success.
"We just didn't turn up with the ball or in the field," O'Keefe conceded.
"That's our worst overall performance as a side. We definitely didn't deserve to win.
"We were well off par with bat and ball and in the field.
"We'll assess how we can do things a bit better. There are no excuses for not getting up, particularly in front of a home crowd."
Next stop for the Sixers is Shane Warne's Melbourne All-Stars at the MCG on Friday night.
O'Keefe can't wait to take on the spin bowling legend for the first time at this level.
"To get to play against Warnie, I still pinch myself," he said.
"I'm an average cricketer here at the Blues and I get to turn up and play against the likes of these guys.
"I am going to be pretty pumped."
Asked if he had a plan to combat Warne's sledging, O'Keefe said with a laugh: "I'm going to try and watch it (the ball) as hard as I can and not listen to any of the crap that comes out of his mouth."