After returning to Penang from England in 1927, she joined the law firm Lim &amp; Lim Advocates and Solicitors. When the war broke out in 1948 and the Japanese invaded Malaya, Oon fled and went to live in Singapore. However, Singapore soon fell into Japanese occupation as well, but sing the Asian population was allowed freedom, Oon decided to utilize that freedom and smuggle and deliver letters to prisoners of war that were held captive in Changi prison.