First Complete said the decision had been mutually agreed on both sides.
O’Brien was formerly the chief executive of Home of Choice before the network was bought out of administration by LSL Property Services for £1.5m and rebranded as First Complete.
LSL group financial services director Jon Round will now take on the additional role of chief executive of First Complete.
Round said: “Having spent the last few weeks finalising the integration of Home of Choice into First Complete, we have mutually agreed to part company with Gerry O’Brien.”
He added: “I would like to thank Gerry for all his help in bringing the business to this point and his hard work in helping to secure the future of the business and the future for all First Complete’s appointed representatives.”
In addition, former commercial director of Home of Choice, John Ahmed, has also agreed to leave First Complete this week.