In many process units, for example, chemical reactors, flammable gases are operated with pure oxygen, and the data of flammability limits in oxygen are crucial to persons who operate such units. Although the data of flammability limits in oxygen are desirable, there is a significant gap between the demand for such data and their availability. In this regard, an effective method for estimating flammability limits in oxygen is indispensible. In this article, formulations based on a thermodynamic approach are proposed to estimate both dui upper flammability limit (UFL) and lower flammability limit (LFL) of pure flammable gases in oxygen at atmospheric pressure and room temperature. The proposed formulations are applicable for flammable gases under constant-pressure combustion. These formulations were then examined by an experimental data set of 21 flammable gases. For these investigated gases, average predictive errors are found to be of 4.94% and 6.67% for predicting LFL and UFL in oxygen, respectively. It is also elucidated that for a flammable gas, which tends to decompose or gives rise to a cool flame, the proposed method may result in larger predictive errors because under such conditions the combustion process should be better explained by the chain theory instead of the thermal theory; it is also shown that if the adiabatic flame temperature is higher than 1850 K, the proposed method also my result in larger predictive errors because of the dissociation of burnt products.