Sexton has been first choice for Ireland since the 2009/10 season but was injured during this year's RBS 6 Nations as O'Gara and then Paddy Jackson both stepped in to replace him at No. 10.
And after the best part of a decade vying with each other, Sexton admits that his old rival's decision to join him in Paris certainly took him by surprise.
"I thought Ronan would go for another season with Munster," Sexton said.
"I thought they'd want to hold on to him. But if he does come over with me it will be great.
"[O'Gara will be] someone to help me settle in and someone I can fall back on who has a lot of experience. I don't know what his role exactly is, we'll have to sit down and have a talk.
"We've developed a really good friendship over the last few years. Obviously we didn't get off on the best note, but we've had a really strong relationship and I really look forward to working with him."