One of the main challenges of modern times is making the energy sector increase its uptake in renewable energy, and determining the role that fossil fuel companies can play in helping or hindering this process. The present study analyses the business strategy of PTT, a state-owned Oil and Gas company in Thailand, and two of its associates, Thai Oil Group and Bangchak Petroleum, to 1) examine renewable energy investment in the past 15 years and 2) shed light on discourses that the companies have used to legitimize their new businesses. For this purpose annual reports from the company websites were analysed, which highlighted how biofuels were the main priority for investment for all three companies since the early 2000s, whereas Solar PV was also recently targeted by PIT and Bangchak Petroleum. The discourses formed to legitimize their investment varied according to energy source and company. Discourses on complying with government policy, enhancing national energy security, and increasing the uptake in environmental friendly energy were found repeatedly in the annual reports of all three companies. Finally, the study provides policy recommendations on how Thai authorities can take a proactive role in helping O&G companies' transition towards a low-carbon energy future.