Seelan Gobalsamy, who has served in senior executive roles at institutions including Liberty Holdings, STANLIB Limited and Old Mutual, will assume the role of CEO with immediate effect.
The company said de Lange stepped down for personal reasons but would remain available in an advisory capacity.
Omnia said it planned to appoint additional non-executive directors in due course in order to support the executive management team responsible for implementing its new strategy.
In August the company said it planned a 2 billion rand ($130 million) rights issue which it will use to reduce debt.
The firm is also evaluating returns from its business units and looking at cost cutting as it battles high debt and thin margins squeezed by difficult trading conditions.
A slowdown in the manufacturing and mining sectors and drought conditions have weighed on Omnia, which produces chemicals and industrial products, fertilisers and explosives.
(Reporting by Tanisha Heiberg; Editing by Alexandra Hudson)
