But the Ireland and Leinster fly-half insisted he is looking forward to working with O'Gara when he moves to Paris next season. O'Gara, 36, has turned down the offer of a contract extension with Munster to take up a two-year deal as kicking and half-backs coach with the wealthy French club.
Sexton told RTE Sport: "I thought he'd go for another season with Munster. I thought they'd want to hold on to him. But if he does come over with me it will be great, someone to help me settle in and someone I can fall back on who has a lot of experience."
He added: "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."
O'Gara won 128 caps for Ireland, toured with the British and Irish Lions on three occasions and won two Heineken Cup titles with Munster in a glittering 16-year career.
He told Sky Sports: "It's really, really difficult to leave it behind but there's a natural cycle to everything and I kind of feel now that I've achieved a lot. I'd love to play on but you have to be realistic. I'm going to do the best I can as a coach.
"It's probably a good time to go, I've experienced every emotion in the game and after beating Harlequins (in the Heineken Cup quarter-final) and then having a difficult day against Clermont (in the semi-final) it felt like it was a good time to leave."
Irish Rugby Football Union chief executive Philip Browne hailed O'Gara as an "iconic figure" and singled out his drop goal that won Ireland the 2009 Grand Slam as "arguably his finest hour".
A statement released by Munster confirming O'Gara's playing retirement said his contribution for the province would never be bettered. "Munster's most decorated player, O'Gara's been capped 240 times in a career that began in August 1997 and saw him steer Munster to Irish Interprovincial, Celtic League, Celtic Cup, Magners League and two Heineken cup titles.
He scored a record 2,625 points, including 1,365 Heineken Cup points to become the all time leading points scorer in that competition, a feat unlikely to be surpassed."
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- Heineken Cup