O'Sullivan is also chair of SiteMinder and Carsales.com, and non-executive director of Afterpay and Calvary Health Care.
He is a former CFO of Optus and Nine Entertainment, and has served on the boards of companies including iiNet, Local Agent Finder, Marley Spoon and Healthengine.
"Today's announcement is the final step of a carefully planned transition, that started with the appointment of our long serving chief operating officer, Mr Edward Chung to the role of CEO in 2017, the renewal of our Board over the last 5 years, and the recruitment of a very experienced deputy chair Mr Pat O'Sullivan," said Di Marco.
"Pat's extensive experience, gives me the confidence he will do an outstanding job in leading the TechnologyOne Board to continue to create substantial wealth for our shareholders," he added.
Di Marco founded TechnologyOne 35 years ago. The venture was funded by the Mactaggart family, which has been involved ever since.
"The idea was to build 'true software products' here in Australia that did not require the source code to be modified, but instead could be configured, with each customer's unique configuration sitting outside the software. Today this is common practice but back then it was revolutionary," he said.
"It has been an amazing journey as we have navigated successfully across four major technology paradigm shifts, starting first with the advent of relational database technology, then the PC, the internet and more recently the cloud. At each stage we have had the conviction and courage to rebuild our products and our business to adapt to a new world.
"Today our global ERP SaaS offering is delivering significant benefits to our customers and fuelling our growth. I see strong growth continuing for many years to come, as we continue to invest strongly in R&D to maintain our market leading position."
He expects the company will "continue to be very successful under the strong leadership of our outstanding CEO Edward Chung and his team.
"In the end, TechnologyOne's success comes down to the amazing hard working and passionate people that work at TechnologyOne, that I have had the honour to work with for so many years. To all of our people I say thank you."