O’Connor, who has worked alongside Cockerill in taking the team to the five Premiership Finals, a Heineken Cup Final and an LV= Cup win since arriving at the club in the summer of 2008, will take over as head coach of Leinster this summer.
Castrogiovanni, the Premiership Player of the Year in 2006/07 in his first season with the club, is also moving on in the summer after 146 appearances for Tigers.
Director of rugby Cockerill said: “We didn’t want to look too far ahead in the middle of the build-up to the Final but I want to thank Matt and Castro – and Geordan Murphy who has had his last game as part of the playing squad – for all they have done for the club.
“Matt has been fantastic for us. We were thrown together really – it was a bit of a forced marriage but it has become a good one.
“He has committed himself whole-heartedly here in his five years with the club. He is a quality coach and I’m sorry to see him go. I have no doubts he will do a great job at Leinster. We wish him well and now we have to fill that hole at Leicester.”
Castrogiovanni, who has 95 caps for Italy, made his last appearance for Tigers in Saturday’s Aviva Premiership Final, collecting a fourth league title in his seven years with the club.
“I’ve got nothing but respect for Castro and what he has done for this club,” said Cockerill.
“He has had seven years here, he’s a great bloke, a great player and we will miss him. He will be remembered as one of the iconic players in the club’s history.
“He is 31 and wants to play as much as he can. We understand that and we wish him well for the future. Now he leaves as a Champion of England and he will always be welcome at Welford Road.”
Murphy has ended his playing career with 322 appearances, 93 tries and 691 points for Tigers, as well as an astonishing collection of honours, including eight Premiership titles, two European Cups, 72 Test caps for Ireland and two Tests for the British & Irish Lions.
“Geordan has been a magnificent servant of the club since he arrived as a teenager 16 years ago,” said Cockerill.
“He is a phenomenal natural talent and he has survived at the very top end of the game for such a long time. He is very popular with supporters and colleagues, and he fully justifies all the accolades that come his way.
“Credit to the players who allowed him and Castro to lift the trophy at Twickenham on Saturday.
“Now Geordan is starting a new career in coaching with us and we look forward to seeing that develop at the Leicester Tigers.”
