Storage of thermal energy by reversible chemical reactions - or thermochemical energy storage - is a promising technology that can play a considerable role in energy systems of the future. In this work, we report the method for discovering thermochemical materials based on the hydration and carbonation of metal oxides. The interpretable basicity descriptor based on local electronic properties of oxygen from DFT computations was found by using the symbolic regression method called SISSO and was shown to predict reaction enthalpy with RMSE 12 kJ/mol which allows the selection of oxides with oxygen potentially reactive in the range of 200-600°C for thermochemical energy storage in industry. We further report screening of oxygen in 320 oxides of earth-abundant metals, selecting several promising candidates for high-density thermochemical energy storage.