"We appreciate Dominic's hard work and service throughout the years and wish him the best for the future," said Crayon CEO Melissa Mulholland. Crayon completed its acquisition of rhipe earlier this month.
O'Hanlon had been CEO of rhipe since 2014.
"Going forward I have asked Mark to lead the rhipe organisation, given his expertise in managing large and complex change projects, and his impressive track record," Mulholland added.
McLellan joined rhipe in 2016 as CFO, and took on the additional role of COO in 2018.
He previously worked for Royal Bank of Scotland, Ernst & Young, and PricewaterhouseCoopers.
McLellan holds an honours degree in economics from the University of Strathclyde (Scotland) and is a member of the Institute of Chartered Accountants of Scotland.
"On behalf of all rhipe employees, I would like to extend our sincere thanks to Dominic for his considerable contributions in shaping the rhipe business and I look forward to furthering our growth because our integration into Crayon will provide more services and innovation," he said.
O'Hanlon said "I am incredibly proud of what has been achieved over the past seven years and have thoroughly enjoyed my time here at rhipe.
"The transformation that rhipe has experienced since this time has been amazing. The foundations have been well built and rhipe is incredibly well positioned for its next stage of evolution, as the transition into the Crayon family begins."