O'Neill will join the UK senior management team, working across Red Havas' London and Manchester offices.
Her appointment follows a period of organic growth for the business in the UK.
In her new role O'Neill will focus on generating new business leads in the UK and EMEA, account management, and expanding the scope of work with current clients.
She will report to Nigel Hughes, managing director of Manchester, and Rachael Sansom, managing director of London, who joined from Red Consultancy last year.
O'Neill has worked on a number of global and UK brands and businesses including Kellogg's, Aldi, and Procter & Gamble. She led the Red Havas team that worked on the highly-acclaimed Greggs Vegan Sausage Roll campaign.
"I'm thrilled to be taking this exciting new step with Red Havas as we drive forward merged media opportunities across both offices," she said.
Sansom said the appointment is a sign of the direction the agency is taking after "a quarter of considerable growth".
"We've got the energy of a start-up with the network of a large agency," Sansom said.
The London boss added that the agency plans to hire more creatives.
"A key initiative is trying to attract more blue-chip clients. Growth has been really organic at the moment, and I can see a moment where we start to look at acquisitions, but that's a bit further down the line."
Since establishing the London office nine months ago, the agency now has nearly 30 staff in the UK.
Red Havas global chief executive and Havas PR Collective chair James Wright said: "Red Havas UK has shown tremendous growth in the past year and we have ambitious plans to further develop our UK operation in the next 12 months."