Aitken teams up with O'Leary at Al Ahli
Aitken had worked with manager Alex McLeish and the club's other first team coach, Andy Watson, since November 2007. He had previously assisted McLeish when he was the manager of the Scottish national side.
Aitken has a previous association with O'Leary, having worked under him at both Leeds United and Aston Villa
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Hide AdMcLeish told bcfc.com: "Naturally I am very disappointed to lose an extremely valued member of Birmingham City Football Club. But he has received an offer that he couldn't refuse and I totally understand the decision he has made for himself and his family.
McLeish added: "Because of his experience and personality, there can be no doubting this leaves a bit of a void but it is one we will work hard to fill as soon as possible."