Oosthuizen has been with the company for 15 years and was responsible for building the agency's PR and Influence practice, growing it from 16 employees into an operation of more than 100 across three offices.
In 2018, she was promoted to COO for Ogilvy South Africa, where she "integrated several operating units to create greater efficiencies within the company, while maintaining client value".
Oosthuizen will be based in London, but will remain active within Ogilvy South Africa.
"Joanna has spearheaded some of our most successful work and built some of our most valuable client relationships. She is also an inspirational leader, who mentored many of our greatest achievers," said Ogilvy South Africa chief executive Alistair Mokoena.