Ostreopsis cf. ovata is a bloom-forming dinoflagellate that has recently been observed in Thai coastal waters. Establishing reliable laboratory culture protocols is essential for conducting physiological, molecular, and toxicological investigations. This study evaluated the growth kinetics and morphological stability of a Thai clonal culture of Ostreopsis cf. ovata across two culture media (IMK and T1) at four nutrient concentrations (1%, 10%, 50%, and 100%). Our findings indicate that the optimal culture conditions are dependent on the specific research objective. The T1 10% medium is the recommended condition for morphological studies as it maintained the highest morphological stability throughout the 30-day experiment, exhibiting the lowest morphological variability (%CV = 2.82%) and the fewest cellular abnormalities. Conversely, T1 50% and IMK 10% facilitated superior biomass production, providing the cell density necessary for other research applications, such as molecular and toxicity studies. For optimal results, morphological analysis should be conducted throughout the experiment, whereas cell harvesting for biomass-dependent applications is advised during the late exponential phase (days 15–18) to maximize yield while minimizing senescence. In contrast, high nutrient concentrations (100%) resulted in significant cellular stress, such as fragile thecal plates and morphological abnormalities, over time. This study provides a foundational framework for researchers to select appropriate culture conditions, thereby facilitating more accurate monitoring and management of this potentially toxic dinoflagellate in the region.