The heterogeneous thermal reaction between carbon disulphide (CS2) and oxygen (O) atoms, producing carbonyl sulphide (OCS), has been investigated under astrophysically relevant conditions in laboratory experiments. We study this reaction at a range of fixed surface temperatures between 15 and 70 K. Following the interaction of the reactants, on a highly oriented pyrolytic graphite surface at a particular surface temperature, we use temperature-programmed desorption coupled with time-of-flight mass spectrometry to determine the yield of OCS. Consequently, our experimental data reveal the temperature dependence of the OCS yield over the aforementioned range of surface temperatures. Our experimental results indicate that thermal O atoms can readily convert CS2 contained in the icy mantles on interstellar grains to OCS, without the need for energetic processing of the ice. Our data set shows that this reaction proceeds at surface temperatures as low as 15 K. Therefore, contrary to previous suggestions, CS2 molecules in icy mantles are perhaps unlikely to account for a substantial proportion of the sulphur content of dense interstellar clouds. Reaction barriers and Arrhenius pre-exponential factors, and hence rate constants, are derived for this surface reaction by fitting our experimental data using a kinetic model. Additionally, this modelling procedure yields values for the desorption energies of O atoms and OCS.