The controversial Australian star lines up at full-back when London Irish tackle Madejski Stadium visitors Northampton.
It comes after his contract with the Australian Rugby Union was torn up following alleged drunken behaviour at Perth Airport earlier this year, with 44 times-capped O'Connor now opting for Irish as his preferred port of call.
"We have a very strong culture at the club and we've worked hard to keep it," Irish rugby director Brian Smith said.
"James understands his responsibilities here, embraces them and our culture.
"From a rugby and intellectual point of view we are getting a current front-line international player and will have that rub-off effect on our squad.
"James went straight in from school into playing international rugby. A lot of us had the opportunity to grow up and make our mistakes out of the limelight, but he hasn't had that opportunity."
O'Connor, who has signed for Irish until the end of this season, will be part of a team eager to bounce back from last weekend's narrow defeat against Newcastle.
"We felt we let a golden opportunity to win a valuable Premiership match slip last weekend, and we want to put in a big performance in front of our supporters," Smith added.
"Northampton are the in-form team at the moment and we will have to be at our best to shut them down.
"It has been a big week for the club with the announcement of James, and we hope we see a big turn- out by our supporters on Sunday to welcome him to the club."
Dean Richards, meanwhile, believes his Newcastle players have "absolutely nothing to lose" when they visit Premiership leaders Saracens on Sunday.
The Falcons have made a solid start to their first campaign back in top-flight English rugby, winning three games from six starts.
And while they will meet a Saracens side on the rebound following last weekend's 41-20 drubbing by Northampton, rugby director Richards is in an upbeat mood.
"It is going to be an interesting game against a quality side, but we are going down there with absolutely nothing to lose," he said.
"They are a talented side with a lot of strength in depth, and despite them losing a series of players for the autumn internationals, they have top-class players coming into the team."
The Newcastle starting line-up features two former Saracens players in wing Noah Cato and centre Adam Powell, and Powell said: "Saracens is a brilliant challenge for us, especially after our win against London Irish last weekend.
"It will be a good opportunity to see where we are at as a team, and we will be looking to go out there and play some good rugby.
"There has been a real buzz in training this week, and after you get a win under your belt there is always an extra zip in the sessions and everyone has a bit more energy."
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