I mainly used red curry paste for the spiciness of this recipe. I put adding Thai peppers, fresh, or pepper flakes as an option. Thai restaurants often add curry paste or peppers for increased spiciness or coconut milk to cool it down. The level of spiciness can vary depending on the recipe and personal preference. Traditional Thai oxtail soup can be moderately spicy due to the inclusion of Thai chilies, but you can adjust the heat by adding fewer chilies or removing the seeds and membranes from the chilies.